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We present radio, optical, near-infrared and spec troscopic observations of the source B0827+525. We consider this source as the best candidate from the Cosmic Lens All-Sky Survey (CLASS) for a 'dark-lens' system or binary radio-loud quasar. The system consists of two radio components with somewhat different spectral indices, separated by 2.815 arcsec. VLBA observations show that each component has substructure on a scale of a few mas. A deep K-band exposure with the W.M. Keck-II Telescope reveals emission near both radio components. The K-band emission of the weaker radio component appears extended, whereas the emission from the brighter radio component is consistent with a point source. Hubble Space Telescope F160W-band observations with the NICMOS instrument confirms this. A redshift of 2.064 is found for the brighter component, using the LRIS instrument on the W.M. Keck-II Telescope. The probability that B0827+525 consists of two unrelated compact flat-spectrum radio sources is similar to3%, although the presence of similar substructure in both component might reduce this.We discuss two alternative scenarios to explain this system: (i) CLASS B0827+525 is a 'dark-lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525 has met all criteria that thus far have in 100% of the cases confirmed a source as an indisputable gravitational lens system. Despite this, no lens galaxy has been detected with m(F160W)less than or equal to 23 mag. Hence, we might have found the first binary radio-loud quasar. At this moment, however, we feel that the 'dark-lens' hypothesis cannot yet be fully excluded.

We present radio, optical, near-infrared and spec troscopic observations of the source B0827+525. We consider this source as the best candidate from the Cosmic Lens All-Sky Survey (CLASS) for a 'dark-lens' system or binary radio-loud quasar. The system consists of two radio components with somewhat different spectral indices, separated by 2.815 arcsec. VLBA observations show that each component has substructure on a scale of a few mas. A deep K-band exposure with the W.M. Keck-II Telescope reveals emission near both radio components. The K-band emission of the weaker radio component appears extended, whereas the emission from the brighter radio component is consistent with a point source. Hubble Space Telescope F160W-band observations with the NICMOS instrument confirms this. A redshift of 2.064 is found for the brighter component, using the LRIS instrument on the W.M. Keck-II Telescope. The probability that B0827+525 consists of two unrelated compact flat-spectrum radio sources is similar to3%, although the presence of similar substructure in both component might reduce this. We discuss two alternative scenarios to explain this system: (i) CLASS B0827+525 is a 'dark-lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525 has met all criteria that thus far have in 100% of the cases confirmed a source as an indisputable gravitational lens system. Despite this, no lens galaxy has been detected with m(F160W)less than or equal to 23 mag. Hence, we might have found the first binary radio-loud quasar. At this moment, however, we feel that the 'dark-lens' hypothesis cannot yet be fully excluded.

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confidence. We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5 Gi;z, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9 less than or similar to(nu(app)/c)less than or similar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative. The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confidence.We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5 Gi;z, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation.On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9 less than or similar to(nu(app)/c)less than or similar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative.The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.

The only way to detect planets around stars at distances greater than or similar to several kpc is by (photometric or astrometric) microlensing (mu L) observations. In this paper, we show that the capability of photometric mu L extends to the detection of signals caused by planets around stars in nearby galaxies (e.g. M31) and that there is no other method that can achieve this. Due to the large crowding, mu L experiments towards M31 can only observe the high-magnification part of a lensing light curve. Therefore, the dominating channel for mu L signals by planets is in distortions near the peak of high-magnification events as discussed by Griest & Safizadeh (1998). We calculate the probability to detect planetary anomalies for mu L experiments towards M31 and find that jupiter-like planets around stars in M31 can be detected. Though the characterization of the planet(s) involved in this signal will be difficult, the absence of such signals can yield strong constraints on the abundance of jupiter-like planets.

The only way to detect planets around stars at distances greater than or similar to several kpc is by (photometric or astrometric) microlensing (mu L) observations. In this paper, we show that the capability of photometric mu L extends to the detection of signals caused by planets around stars in nearby galaxies (e.g. M31) and that there is no other method that can achieve this. Due to the large crowding, mu L experiments towards M31 can only observe the high-magnification part of a lensing light curve. Therefore, the dominating channel for mu L signals by planets is in distortions near the peak of high-magnification events as discussed by Griest & Safizadeh (1998). We calculate the probability to detect planetary anomalies for mu L experiments towards M31 and find that jupiter-like planets around stars in M31 can be detected. Though the characterization of the planet(s) involved in this signal will be difficult, the absence of such signals can yield strong constraints on the abundance of jupiter-like planets.

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47(-6)(+5), d (68%) or 47(-9)(+12) d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter H(0)=57(-11)(+14) (95% statistical) (+26)(-15) (systematic) km s(-1) Mpc(-1) (Omega(m)=1 and Omega(Lambda)=0). Similarly, the Modified-Hubble-Profile mass model would give H(0)=74(-15)(+18) (95% statistical) (+22)(-22) (systematic) km s(-1) Mpc(-1) For Omega(m)=0.3 and Omega(Lambda)=0.7, these values increase by 5.4%. We emphasize that the slope of the radial mass profile of the lens-galaxy dark-matter halo in B1600+434 is extremely ill-constrained. Hence, an accurate determination of Ho from this system is very difficult, if no additional constraints on the mass model are obtained. These values of Ho should therefore be regarded as indicative. Once H(0) (from independent methods) and the time delay have been determined with sufficient accuracy, it will prove more worthwhile to constrain the radial mass profile of the dark-matter halo around the edge-on spiral lens galaxy at z=0.4.

We present constraints on the cosmological constant lambda(0) and the density parameter Omega(0) from joint constraints from the analyses of gravitational lensing statistics of the Jo- drell Bank-VLA Astrometric Survey (JVAS), optical gravitational lens surveys from the literature and CMB anisotropies. This is the first time that quantitative joint constraints involving lensing statistics and CMB anisotropies have been presented. Within the assumptions made, we achieve very tight constraints on both lambda(0) and Omega(0). These assumptions are cold dark matter models, no tensor components, no reionisation, CMB temperature T-CMB = 2.728 K, number of neutrinos n(nu) = 3, helium abundance Y-HE = 0.246, spectral index n(s) = 1.0, Hubble constant H-0 = 68 km s(-1) Mpc(-1), baryonic density Omega(b) = 0.05. All models were normalised to the COBE data and no closed models (k = +1) were computed. Using the CMB data alone, the best-fit model has lambda(0) = 0.60 and lambda(0) = 0.34 and at 99% confidence the lower limit on lambda(0) + Omega(0) is 0.8. Including constraints from gravitational lensing statistics doesn't change this significantly, although it does change the allowed region of parameter space. A universe with lambda(0) = 0 is ruled out for any value of Omega(0) at better than 99% confidence using the CMB alone. Combined with constraints from lensing statistics, lambda(0) = 0 is also ruled out at better than 99% confidence.As the region of parameter space allowed by the CMB is, within our assumptions, much smaller than that allowed by lensing statistics, the main result of combining, a the two is to change the range of parameter space allowed by the CMB along its axis of degeneracy.

We present constraints on the cosmological constant lambda(0) and the density parameter Omega(0) from joint constraints from the analyses of gravitational lensing statistics of the Jo- drell Bank-VLA Astrometric Survey (JVAS), optical gravitational lens surveys from the literature and CMB anisotropies. This is the first time that quantitative joint constraints involving lensing statistics and CMB anisotropies have been presented. Within the assumptions made, we achieve very tight constraints on both lambda(0) and Omega(0). These assumptions are cold dark matter models, no tensor components, no reionisation, CMB temperature T-CMB = 2.728 K, number of neutrinos n(nu) = 3, helium abundance Y-HE = 0.246, spectral index n(s) = 1.0, Hubble constant H-0 = 68 km s(-1) Mpc(-1), baryonic density Omega(b) = 0.05. All models were normalised to the COBE data and no closed models (k = +1) were computed. Using the CMB data alone, the best-fit model has lambda(0) = 0.60 and lambda(0) = 0.34 and at 99% confidence the lower limit on lambda(0) + Omega(0) is 0.8. Including constraints from gravitational lensing statistics doesn't change this significantly, although it does change the allowed region of parameter space. A universe with lambda(0) = 0 is ruled out for any value of Omega(0) at better than 99% confidence using the CMB alone. Combined with constraints from lensing statistics, lambda(0) = 0 is also ruled out at better than 99% confidence. As the region of parameter space allowed by the CMB is, within our assumptions, much smaller than that allowed by lensing statistics, the main result of combining, a the two is to change the range of parameter space allowed by the CMB along its axis of degeneracy.

We have developed a new technique for weak lensing analysis, specifically designed to correct accurately for the effect of the point spread function (PSF) on the shapes of small galaxy images. This method does not rely on weighted second moments of detected images, which we show can leave residuals at the level of a percent in the shear; rather, we directly fit observed galaxy images as PSF-convolved, sheared circular sources. We show by means of simulations that this technique is able to recover shears with systematic errors well below the percent level for representative PSF shapes, while its noise properties are similar to existing methods.

We have developed a new technique for weak lensing analysis, specifically designed to correct accurately for the effect of the point spread function (PSF) on the shapes of small galaxy images. This method does not rely on weighted second moments of detected images, which we show can leave residuals at the level of a percent in the shear; rather, we directly fit observed galaxy images as PSF-convolved, sheared circular sources. We show by means of simulations that this technique is able to recover shears with systematic errors well below the percent level for representative PSF shapes, while its noise properties are similar to existing methods.

We discuss the statistical properties of faint background galaxies detected in a deep Hubble Space Telescope ultraviolet (F336W) Wide Field Planetary Camera 2 image of the lensing cluster Abell 370 (z=0.37). By combining modelled field galaxy counts at this wavelength with a detailed mass model for the cluster, we develop techniques for predicting the expected redshift distribution of lensed sources and find the bulk of the sources should be moderately magnified sources whose redshifts lie between 0.5 and 2. We compare these predictions with redshift estimates derived from the lens inversion method which utilises the shapes of individual arclets in the context of a known field ellipticity distribution. This allows us to infer the comoving star formation rate for the range 0.5

We discuss the statistical properties of faint background galaxies detected in a deep Hubble Space Telescope ultraviolet (F336W) Wide Field Planetary Camera 2 image of the lensing cluster Abell 370 (z=0.37). By combining modelled field galaxy counts at this wavelength with a detailed mass model for the cluster, we develop techniques for predicting the expected redshift distribution of lensed sources and find the bulk of the sources should be moderately magnified sources whose redshifts lie between 0.5 and 2. We compare these predictions with redshift estimates derived from the lens inversion method which utilises the shapes of individual arclets in the context of a known field ellipticity distribution. This allows us to infer the comoving star formation rate for the range 0.5

I present constraints on cosmological parameters in the lambda(0)-Omega(0) plane from a joint analysis of gravitational lensing statistics (Helbig et al. 1999b) and the magnitude-redshift relation for Type Ia supernovae (Perlmutter et al. 1999; Riess et al. 1998). I discuss reasons why this particular combination of tests is important and how the constraints can be improved in the future. The lensing statistics and supernova results are not inconsistent, thus it is meaningful to determine joint constraints on lambda(0) and Omega(0) by combining the results from both tests. The quantity measured by the lens statistics and the m-z relation for type Ia supernovae discussed here is approximately lambda(0) - Omega(0). At 95% confidence, the upper limit on lambda(0) - Omega(0) from lensing statistics alone is 0.45 and from supernovae alone is in the range 0.65-0.81 (depending on the data set). For joint constraints, the upper limit on lambda(0) - Omega(0) is in the range .55-0.60 (again depending on the data set). For a flat universe with lambda(0) + Omega(2) = 1, this corresponds to upper limits on lambda(0), taking the top of the range from different data sets, of 0.72, 0.90 and 0.80 for lensing statistics alone, supernovae alone and the joint analysis, respectively. This is perfectly consistent with the current 'standard cosmological model' with lambda(0) approximate to 0.7 and Omega(0) approximate to 0.3 and is consistent with a flat universe but, neglecting other cosmological tests, does riot require it.

I present constraints on cosmological parameters in the lambda(0)-Omega(0) plane from a joint analysis of gravitational lensing statistics (Helbig et al. 1999b) and the magnitude-redshift relation for Type Ia supernovae (Perlmutter et al. 1999; Riess et al. 1998). I discuss reasons why this particular combination of tests is important and how the constraints can be improved in the future. The lensing statistics and supernova results are not inconsistent, thus it is meaningful to determine joint constraints on lambda(0) and Omega(0) by combining the results from both tests. The quantity measured by the lens statistics and the m-z relation for type Ia supernovae discussed here is approximately lambda(0) - Omega(0). At 95% confidence, the upper limit on lambda(0) - Omega(0) from lensing statistics alone is 0.45 and from supernovae alone is in the range 0.65-0.81 (depending on the data set). For joint constraints, the upper limit on lambda(0) - Omega(0) is in the range .55-0.60 (again depending on the data set). For a flat universe with lambda(0) + Omega(2) = 1, this corresponds to upper limits on lambda(0), taking the top of the range from different data sets, of 0.72, 0.90 and 0.80 for lensing statistics alone, supernovae alone and the joint analysis, respectively. This is perfectly consistent with the current 'standard cosmological model' with lambda(0) approximate to 0.7 and Omega(0) approximate to 0.3 and is consistent with a flat universe but, neglecting other cosmological tests, does riot require it.

The transition of the binary gravitational lens from the equal mass case to small (planetary) mass ratios q is studied. It is shown how the Limit of a (pure shear) Chang-Refsdal lens is approached, under what conditions the Chang-Refsdal approximation is valid, and how the 3 different topologies of the critical curves and caustics for a binary lens are mapped onto the 2 different topologies for a Chang-Refsdal lens with pure shear. It is shown that for wide binaries, the lensing in the vicinity of both lens objects can be described by a Taylor-expansion of the deflection term due to the other object, where the Chang-Refsdal approximation corresponds to a truncation of this series. For close binaries, only the vicinity of the secondary, less massive, object can be described in this way. However, for image distances much larger than the separation of the lens objects, any binary lens can be approximated by means of multipole expansion, where the first non-trivial term is the quadrupole term. It is shown that an ambiguity exists between wide and close binary lenses, where the shear at one of the objects due to the other object for the wide binary is equal to the absolute value of the eigenvalues of the quadrupole moment for the close binary. This analysis provides the basis for a classification of binary lens microlensing events, especially of planetary events, and an understanding of present ambiguities.

The transition of the binary gravitational lens from the equal mass case to small (planetary) mass ratios q is studied. It is shown how the Limit of a (pure shear) Chang-Refsdal lens is approached, under what conditions the Chang-Refsdal approximation is valid, and how the 3 different topologies of the critical curves and caustics for a binary lens are mapped onto the 2 different topologies for a Chang-Refsdal lens with pure shear. It is shown that for wide binaries, the lensing in the vicinity of both lens objects can be described by a Taylor-expansion of the deflection term due to the other object, where the Chang-Refsdal approximation corresponds to a truncation of this series. For close binaries, only the vicinity of the secondary, less massive, object can be described in this way. However, for image distances much larger than the separation of the lens objects, any binary lens can be approximated by means of multipole expansion, where the first non-trivial term is the quadrupole term. It is shown that an ambiguity exists between wide and close binary lenses, where the shear at one of the objects due to the other object for the wide binary is equal to the absolute value of the eigenvalues of the quadrupole moment for the close binary. This analysis provides the basis for a classification of binary lens microlensing events, especially of planetary events, and an understanding of present ambiguities.

We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shed new light on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm the multiple image nature of the B2-B3 gravitational pair (Kneib et al. 1993), and determine a redshift of z = 0.806 +/- 0.002 which is in very good agreement with earlier predictions. A refined mass model of the cluster core (that includes cluster galaxy halos) is presented, based on a number of newly identified multiple images. Following Bezecourt et al. (1998a), we combine the new cluster mass model with a spectrophotometric prescription for galaxy evolution to predict the arclets number counts and redshift distribution in the I-IST image. In particular, the ellipticity distribution of background sources is taken into account, in order to properly estimate the statistical number and redshift distribution of arclets. We show that the redshift distribution of arclets, and particularly its high redshift tail can be used as a strong constraint to disentangle different galaxy evolution scenarios. A hierarchical model which includes number density evolution is favored by our analysis. Finally, we compute the depletion curves :For the faint galaxy number counts and discuss their wavelength dependence.

We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shed new light on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm the multiple image nature of the B2-B3 gravitational pair (Kneib et al. 1993), and determine a redshift of z = 0.806 +/- 0.002 which is in very good agreement with earlier predictions. A refined mass model of the cluster core (that includes cluster galaxy halos) is presented, based on a number of newly identified multiple images. Following Bezecourt et al. (1998a), we combine the new cluster mass model with a spectrophotometric prescription for galaxy evolution to predict the arclets number counts and redshift distribution in the I-IST image. In particular, the ellipticity distribution of background sources is taken into account, in order to properly estimate the statistical number and redshift distribution of arclets. We show that the redshift distribution of arclets, and particularly its high redshift tail can be used as a strong constraint to disentangle different galaxy evolution scenarios. A hierarchical model which includes number density evolution is favored by our analysis. Finally, we compute the depletion curves :For the faint galaxy number counts and discuss their wavelength dependence.

We present the first results on the identification and study of very distant field galaxies in the core of cluster-lenses, using a selection criterium based on both lens modelling and photometric redshifts. We concentrate on two multiple-imaged sources at z = 4.05 in the cluster Abell 2390. The two objects presented in this paper, namely H3 (cusp are) and H5 (fold are), were identified through lens modelling as multiple images of high-redshift sources at z greater than or similar to 3.5 (Kneib et al. 1999). We confirm the excellent agreement between this identification and both their photometric redshifts and morphologies. Our CFHT/WHT program for a systematic redshift survey of arcs in clusters has allowed to obtain a set of spectra on three different images at z similar to 4: the brightest image of H3, whose redshift was already confirmed by Frye & Broadhurst (1998), and the two brightest images of H5. The later is then confirmed spectroscopically as a multiple image, giving a strong support to the lens model. The main feature in each of these spectra is a strong emission line, identified as Ly a, leading to z. = 4.05 for both H3 and H5. The spectrophotometric properties of these galaxies are studied, in particular the degeneracy in the parameter-space defined by the SFR type, age, metallicity and reddening. H3 and H5 are intrinsically bright and clumpy galaxies (M-B* to M-B* -2 magnitudes), located similar to 100h(50)(-1) kpc apart on the source plane, with mean metallicities compatible with a fraction of solar or even solar values. These results seem to favour a hierarchical merging scenario, when we are seeing a relatively evolved phase in these two z similar to 4 objects, with stars forming locally and efficiently.

We present the first results on the identification and study of very distant field galaxies in the core of cluster-lenses, using a selection criterium based on both lens modelling and photometric redshifts. We concentrate on two multiple-imaged sources at z = 4.05 in the cluster Abell 2390. The two objects presented in this paper, namely H3 (cusp are) and H5 (fold are), were identified through lens modelling as multiple images of high-redshift sources at z greater than or similar to 3.5 (Kneib et al. 1999). We confirm the excellent agreement between this identification and both their photometric redshifts and morphologies. Our CFHT/WHT program for a systematic redshift survey of arcs in clusters has allowed to obtain a set of spectra on three different images at z similar to 4: the brightest image of H3, whose redshift was already confirmed by Frye & Broadhurst (1998), and the two brightest images of H5. The later is then confirmed spectroscopically as a multiple image, giving a strong support to the lens model. The main feature in each of these spectra is a strong emission line, identified as Ly a, leading to z. = 4.05 for both H3 and H5. The spectrophotometric properties of these galaxies are studied, in particular the degeneracy in the parameter-space defined by the SFR type, age, metallicity and reddening. H3 and H5 are intrinsically bright and clumpy galaxies (M-B* to M-B* -2 magnitudes), located similar to 100h(50)(-1) kpc apart on the source plane, with mean metallicities compatible with a fraction of solar or even solar values. These results seem to favour a hierarchical merging scenario, when we are seeing a relatively evolved phase in these two z similar to 4 objects, with stars forming locally and efficiently.

We present a study of a very peculiar object found in the field of the cluster-lens Abell 370. This object displays, in HST imaging, a spectacular morphology comparable to nearby ring-galaxies. From spectroscopic observations at the CFHT, we measured a redshift of z = 1.062 based on the identification of [OII] 3727 Angstrom and [Nev] 3426 Angstrom emission lines. These emission lines are typical of starburst galaxies hosting a central active nucleus and are in good agreement with the assumption that this object is a ring galaxy. This object is also detected with ISO in the LW2 and LW3 filters, and the mid Infra-Red (MIR) flux ratio favors a Seyfert 1 type. The shape of the ring is gravitationally distorted by the cluster-lens, and in particular by a nearby cluster elliptical galaxy. Using our cluster mass model, we can compute its intrinsic shape. Requiring that the outer ring follows an ellipse we constrain the M/L ratio of the nearby galaxy and derive a magnification factor of 2.5 +/- 0.2. The absolute luminosities of the source are then L-B = 1.3 10(12) L-B. and upsilon L-upsilon similar or equal to 4.10(10) L. in the mid-IR.

We present a study of a very peculiar object found in the field of the cluster-lens Abell 370. This object displays, in HST imaging, a spectacular morphology comparable to nearby ring-galaxies. From spectroscopic observations at the CFHT, we measured a redshift of z = 1.062 based on the identification of [OII] 3727 Angstrom and [Nev] 3426 Angstrom emission lines. These emission lines are typical of starburst galaxies hosting a central active nucleus and are in good agreement with the assumption that this object is a ring galaxy. This object is also detected with ISO in the LW2 and LW3 filters, and the mid Infra-Red (MIR) flux ratio favors a Seyfert 1 type. The shape of the ring is gravitationally distorted by the cluster-lens, and in particular by a nearby cluster elliptical galaxy. Using our cluster mass model, we can compute its intrinsic shape. Requiring that the outer ring follows an ellipse we constrain the M/L ratio of the nearby galaxy and derive a magnification factor of 2.5 +/- 0.2. The absolute luminosities of the source are then L-B = 1.3 10(12) L-B. and upsilon L-upsilon similar or equal to 4.10(10) L. in the mid-IR.

For observed galactic microlensing events only one fit is usually presented, though, especially for a binary lens, several fits may be possible. This has been shown for the MACHO LMC#1 event (Dominik & Hirshfeld 1996). Here I discuss the strong binary lens events OGLE#7 and DUO#2. It is shown that several models with a large variety of parameters are in accordance with the photometric data. For most of the fits, 1-sigma-bounds on the fit parameters are given. The variation of the parameters within these bounds is in some cases considerable. It is likely that other binary lens events which will occur will have properties similar to the discussed events.

For observed galactic microlensing events only one fit is usually presented, though, especially for a binary lens, several fits may be possible. This has been shown for the MACHO LMC#1 event (Dominik & Hirshfeld 1996). Here I discuss the strong binary lens events OGLE#7 and DUO#2. It is shown that several models with a large variety of parameters are in accordance with the photometric data. For most of the fits, 1-sigma-bounds on the fit parameters are given. The variation of the parameters within these bounds is in some cases considerable. It is likely that other binary lens events which will occur will have properties similar to the discussed events.

We present the first unambiguous case of external variability of a radio gravitational lens, CLASS B1600+434. The Very Lai-ge Array (VLA) 8.5-GHz difference light curve of the lensed images, taking the proper time-delay into account, shows the presence of external variability with 14.6-sigma confidence. We investigate two plausible causes of this external variability: scattering by the ionized component of the Galactic interstellar medium and microlensing by massive compact objects in the bulge/disk and halo of the lens galaxy. Based on the tight relation between the modulation-index (fractional rms variability) and variability time scale and the quantitative difference between the light curves of both lensed images, we conclude that the observed short-term variability characteristics of the lensed images are incompatible with scintillation in our Galaxy. This conclusion is strongly supported by multi-frequency Westerbork Synthesis Radio Telescope (WSRT) observations at 1.4 and 5 Gi;z, which are in disagreement with predictions based on the scintillation hypothesis. Several arguments against scintillation might need to be reevaluated if evidence is found for significant scatter-broadening of lensed image B seen through the lens galaxy. However, the frequency-dependence and time scale of variability from image A are not affected by this and remain strong arguments against scintillation. On the other hand, a single superluminal jet-component in the source, having an apparent velocity 9 less than or similar to(nu(app)/c)less than or similar to 26, a radius of 2-5 mu as and containing 5-11 % of the observed 8.5-GHz source flux density, can reproduce the observed modulation-indices and variability time scale at 8.5 GHz, when it is microlensed by compact objects in the lens galaxy. It also reproduces the frequency-dependence of the modulation-indices, determined from the independent WSRT 1.4 and 5-GHz observations. The difference between the modulation-indices of the lensed images (i.e. 2.8% and 1.6% at 8.5 GHz in 1998 for images A and B, respectively), if not affected by scatter-broadening of image B by the ionized ISM of the lens galaxy, can be explained through a different mass-function for the compact objects in the bulge/disk and halo of the lens galaxy. Comparing the observations with microlensing simulations, we place a tentative lower limit of greater than or similar to 0.5 M. on the average mass of compact objects in the halo line-of-sight. The above-mentioned set of mass-function and source parameters is consistent, although not unique, and should only be regarded as indicative. The only conclusion fully consistent with the data gathered thus far is that we have indeed detected radio microlensing. The far reaching consequence of this statement is that a significant fraction of the mass in the dark-matter halo at similar to 6kpc (h=0.65) above the lens-galaxy disk in B1600+434 consists of massive compact objects.

We present constraints on the cosmological constant lambda(0) and the density parameter Omega(0) from joint constraints from the analyses of gravitational lensing statistics of the Jo- drell Bank-VLA Astrometric Survey (JVAS), optical gravitational lens surveys from the literature and CMB anisotropies. This is the first time that quantitative joint constraints involving lensing statistics and CMB anisotropies have been presented. Within the assumptions made, we achieve very tight constraints on both lambda(0) and Omega(0). These assumptions are cold dark matter models, no tensor components, no reionisation, CMB temperature T-CMB = 2.728 K, number of neutrinos n(nu) = 3, helium abundance Y-HE = 0.246, spectral index n(s) = 1.0, Hubble constant H-0 = 68 km s(-1) Mpc(-1), baryonic density Omega(b) = 0.05. All models were normalised to the COBE data and no closed models (k = +1) were computed. Using the CMB data alone, the best-fit model has lambda(0) = 0.60 and lambda(0) = 0.34 and at 99% confidence the lower limit on lambda(0) + Omega(0) is 0.8. Including constraints from gravitational lensing statistics doesn't change this significantly, although it does change the allowed region of parameter space. A universe with lambda(0) = 0 is ruled out for any value of Omega(0) at better than 99% confidence using the CMB alone. Combined with constraints from lensing statistics, lambda(0) = 0 is also ruled out at better than 99% confidence. As the region of parameter space allowed by the CMB is, within our assumptions, much smaller than that allowed by lensing statistics, the main result of combining, a the two is to change the range of parameter space allowed by the CMB along its axis of degeneracy.

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47^{+5}_{-6} d (68%) or 47^{+12}_{-9} d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11} (95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED), 14 pages, 6 figures, accepted for publication in Astronomy & Astrophysics (Figs 1 and 3 with degraded resolution)

The transition of the binary gravitational lens from the equal mass case to small (planetary) mass ratios q is studied. It is shown how the limit of a (pure shear) Chang-Refsdal lens is approached, under what conditions the Chang-Refsdal approximation is valid, and how the 3 different topologies of the critical curves and caustics for a binary lens are mapped onto the 2 different topologies for a Chang-Refsdal lens with pure shear. It is shown that for wide binaries, the lensing in the vicinity of both lens objects can be described by a Taylor-expansion of the deflection term due to the other object, where the Chang-Refsdal approximation corresponds to a truncation of this series. For close binaries, only the vicinity of the secondary, less massive, object can be described in this way. However, for image distances much larger than the separation of the lens objects, any binary lens can be approximated by means of multipole expansion, where the first non-trivial term is the quadrupole term. It is shown that an ambiguity exists between wide and close binary lenses, where the shear at one of the objects due to the other object for the wide binary is equal to the absolute value of the eigenvalues of the quadrupole moment for the close binary. This analysis provides the basis for a classification of binary lens microlensing events, especially of planetary events, and an understanding of present ambiguities., 20 pages in LaTeX2e format with 9 embedded PostScript figures; figures modified and embedded; accepted for publication in A&A

We present a study of a very peculiar object found in the field of the cluster-lens Abell 370. This object displays, in HST imaging, a spectacular morphology comparable to nearby ring-galaxies. From spectroscopic observations at the CFHT, we measured a redshift of $z=1.062$ based on the identification of [O ii] 3727 \AA and [Ne v] 3426 \AA emission lines. These emission lines are typical of starburst galaxies hosting a central active nucleus and are in good agreement with the assumption that this object is a ring-galaxy. This object is also detected with ISO in the LW2 and LW3 filters, and the mid Infra-Red (MIR) flux ratio favors a Seyfert 1 type. The shape of the ring is gravitationally distorted by the cluster-lens, and most particularly by a nearby cluster elliptical galaxy. Using the cluster mass model, we can compute its intrinsic shape. Requiring that the outer ring follows an ellipse we put constraints on the M/L ratio of the nearby galaxy and derive a magnification factor of 2.5 $\pm$ 0.2. The absolute luminosities of the source are then $L_B = 1.3 \ 10^{12} L_{B \odot}$ and $\nu$ L$_\nu \simeq 4. 10^{10}$ L$_\odot$ in the mid-IR., Comment: 5 pages, 5 figures, uses aa.cls, accepted to A&A Letters. Minor changes, Figure 1 revisited and typos added

We discuss the statistical properties of faint background galaxies detected in a deep Hubble Space Telescope ultraviolet (F336W) Wide Field Planetary Camera 2 image of the lensing cluster Abell 370 (z=0.37). By combining modelled field galaxy counts at this wavelength with a detailed mass model for the cluster, we develop techniques for predicting the expected redshift distribution of lensed sources and find the bulk of the sources should be moderately magnified sources whose redshifts lie between 0.5 and 2. We compare these predictions with redshift estimates derived from the lens inversion method which utilises the shapes of individual arclets in the context of a known field ellipticity distribution. This allows us to infer the comoving star formation rate for the range 0.5, Comment: 10 pages, submitted to Astronomy and Astrophysics

For observed galactic microlensing events only one fit is usually presented, though, especially for a binary lens, several fits may be possible. This has been shown for the MACHO LMC#1 event (Dominik & Hirshfeld 1996). Here I discuss the strong binary lens events OGLE#7 and DUO#2. It is shown that several models with a large variety of parameters are in accordance with the photometric data. For most of the fits, 1-sigma-bounds on the fit parameters are given. The variation of the parameters within these bounds is in some cases considerable. It is likely that other binary lens events which will occur will have properties similar to the discussed events.

The only way to detect planets around stars at distances of several kpc is by (photometric or astrometric) microlensing observations. In this paper, we show that the capability of photometric microlensing extends to the detection of signals caused by planets around stars in nearby galaxies (e.g. M31) and that there is no other method that can achieve this. Due to the large crowding, microlensing experiments towards M31 can only observe the high-magnification part of a lensing light curve. Therefore, the dominating channel for microlensing signals by planets is in distortions near the peak of high-magnification events as discussed by Griest and Safizadeh. We calculate the probability to detect planetary anomalies for microlensing experiments towards M31 and find that jupiter-like planets around stars in M31 can be detected. Though the characterization of the planet(s) involved in this signal will be difficult, the absence of such signals can yield strong constraints on the abundance of jupiter-like planets., Comment: 16 LaTex Pages, including 1 Postscript Figures, submitted to A&A; title changed, one more author added, completely revised version: central point is detecting planet in nearby galaxies and one more technique is taken into consideration

We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shade new lights on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm the multiple image nature of the B2--B3 gravitational pair (Kneib et al. 1993), and determine a redshift of z=0.806 which is in very good agreement with earlier predictions. A refined mass model of the cluster core (that includes cluster galaxy halos) is presented, based on a number of newly identified multiple images. Following Bezecourt et al. (1998a), we combine the new cluster mass model with a spectrophotometric prescription for galaxy evolution to predict the arclets number counts and redshift distribution in the HST image. In particular, the ellipticity distribution of background sources is taken into account, in order to properly estimate the statistical number and redshift distribution of arclets. We show that the redshift distribution of arclets, and particularly its high redshift tail can be used as a strong constraint to disentangle different galaxy evolution scenario. A hierarchical model which includes a number density evolution is favored by our analysis. Finally, we compute the depletion curves in the faint galaxies number counts and discuss its wavelength dependence., 10 pages, Astronomy and Astrophysics in press

We have developed a new technique for weak lensing analysis, with which the effect of the point spread function (PSF) on small galaxy images can be corrected for accurately. Rather than relying on weighted second moments of detected images, which we show can leave residuals at the level of a percent in the shear, we directly fit (stacked or individual) galaxy images as PSF-convolved, sheared circular sources. We show by means of simulations that this technique is able to recover shears well below the percent level for a variety of PSF shapes, and that its noise properties are similar to existing methods., 11 pages, A&A, submitted

We present radio, optical, near-infrared and spectroscopic observations of the source B0827+525. We consider this source as the best candidate from the Cosmic Lens All-Sky Survey (CLASS) for a `dark lens' system or binary radio-loud quasar. The system consists of two radio components with somewhat different spectral indices, separated by 2.815 arcsec. VLBA observations show that each component has substructure on a scale of a few mas. A deep K-band exposure with the W.M.Keck-II Telescope reveals emission near both radio components. The K-band emission of the weaker radio component appears extended, whereas the emission from the brighter radio component is consistent with a point source. Hubble Space Telescope F160W-band observations with the NICMOS instrument confirms this. A redshift of 2.064 is found for the brighter component, using the LRIS instrument on the W.M.Keck-II Telescope. The probability that B0827+525 consists of two unrelated compact flat-spectrum radio sources is ~3%, although the presence of similar substructure in both component might reduce this. We discuss two scenarios to explain this system: (i) CLASS B0827+525 is a `dark lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525 has met all criteria that thus far have in 100% of the cases confirmed a source as an indisputable gravitational lens system. Despite this, no lens galaxy has been detected with m_F160W, 9 pages, 6 figures; Accepted for publication in Astronomy & Astrophysics; Full-res. images 1 and 3 can be obtained from L.V.E.K

I present constraints on cosmological parameters in the lambda_0-Omega_0 plane from a joint analysis of gravitational lensing statistics (astro-ph/9904175) and the magnitude-redshift relation for Type Ia supernovae (astro-ph/9812133 and astro-ph/9805201). I discuss reasons why this particular combination of tests is important and how the constraints can be improved in the future. The lensing statistics and supernova results are not inconsistent, thus it is meaningful to determine joint constraints on lambda_0 and Omega_0 by combining the results from both tests. The quantity measured by the lens statistics and the m-z relation for type Ia supernovae discussed here is approximately lambda_0 - Omega_0. At 95% confidence, the upper limit on lambda_0 - Omega_0 from lensing statistics alone is 0.45 and from supernovae alone is in the range 0.65--0.81 (depending on the data set). For joint constraints, the upper limit on lambda_0 - Omega_0 is in the range 0.55--0.60 (again depending on the data set). For a flat universe with lambda_0 + Omega_0 = 1, this corresponds to upper limits on lambda_0, taking the top of the range from different data sets, of 0.72, 0.90 and 0.80 for lensing statistics alone, supernovae alone and the joint analysis, respectively. This is perfectly consistent with the current `standard cosmological model' with lambda_0 approximately 0.7 and Omega_0 approximately 0.3 and is consistent with a flat universe but, neglecting other cosmological tests, does not require it., Comment: LaTeX, 8 pages, 18 PostScript files in 4 figures. Accepted by Astronomy and Astrophysics. After-proof-correction version. Changes from previous version: references updated and corrected, minor typos corrected. Paper version available on request. Data available from http://gladia.astro.rug.nl:8000/ceres/data_from_papers/papers.html or http://multivac.jb.man.ac.uk:8000/ceres/data_from_papers/papers.html