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We redetermine the relationship between absolute magnitude and orbital period for dwarf novae, based on 46 stars with good distance estimates. This improves upon Warner's previous relation, building upon today's improved estimates of distance and binary inclination, and greater wavelength coverage. Together with other distance and dynamical constraints, this calibration is then applied to a set of ~300 known or likely dwarf novae of short orbital period, to study the dependence of quiescent M_v, time-averaged M_v, mass ratio q, and white-dwarf temperature T_WD, on P_orb. These distributions show that stars become much fainter as they approach minimum P_orb, and appear to show /evolutionary tracks/ as the secondary is whittled down by mass loss. Stars on the lower branch have the expected properties of "period bouncers" -- with a feeble secondary, faint accretion light, cool white dwarf, and long recurrence time between eruptions. Period bounce seems to occur at a mass of 0.058+-0.008 M_sol. Stars on the lower branch may also have higher velocities and heights above the Galactic plane, consistent with a greater age. Some are very nearby, despite strong selection effects discriminating against the discovery of these faint binaries accreting at very low rates. Period bouncers appear to be very common, and probably would dominate a complete census of cataclysmic variables., Comment: PDF, 57 pages, 5 tables, 13 figures; accepted, in press, MNRAS; supplementary electronic material, online table; more info at http://cbastro.org/, http://cbastro.org/dwarfnovashort/

We present analysis and results of a coordinated CCD photometry campaign to observe the 2006 June superoutburst of the cataclysmic variable V1316 Cyg involving 8 longitudinally-distributed observers. The outburst peaked at magnitude 15.03 on June 10, declined at a rate of 0.14 mag/day, lasted 11 days and had an amplitude above quiescence of 2.4 magnitudes. We detected common superhumps for the first time, thereby confirming that V1316 Cyg is a member of the UGSU class of dwarf novae. We observed a transition to late superhumps two-thirds of the way through the outburst with an associated phase shift of 0.50 +/- 0.06 cycles. The mean common superhump period before this transition was 0.07685 +/- 0.00003 d and the mean late superhump period following the transition was 0.07654 +/- 0.00002 d. The common superhump period decreased at a rate dP/dt = -5.1 +/- 1.7 x10^-5 /cycle. At the onset of late superhumps, there was a transient shift in power from the superhump fundamental frequency to its first harmonic and back again. We detected an orbital period of 0.0740 +/- 0.0002 d giving a fractional superhump period excess of 0.038 +/- 0.003 and a mass ratio of 0.167 +/- 0.010. A scalegram analysis of the flickering behaviour of V1316 Cyg found that the alpha and sigma parameters characterising flickering changed significantly during the superoutburst. We also found flickering to be at a relatively much lower level at the beginning of the superoutburst and during two normal outbursts., Comment: Accepted for publication in the Journal of the British Astronomical Association, 18 pages, 12 figures

We report the serendipitous detection of a very bright, very nearby microlensing event. In late October 2006, an otherwise unremarkable A0 star at a distance ~1 kpc (GSC 3656-1328) brightened achromatically by a factor of nearly 40 over the span of several days and then decayed in an apparently symmetrical way. We present a light curve of the event based on optical photometry from the Center for Backyard Astrophysics and the All Sky Automated Survey, as well as near-infrared photometry from the Peters Automated Infrared Imaging Telescope. This light curve is well-fit by a generic microlensing model. We also report optical spectra, and Swift X-ray and UV observations that are consistent with the microlensing interpretation. We discuss and reject alternative explanations for this variability. The lens star is probably a low-mass star or brown dwarf, with a relatively high proper motion of >20 mas/yr, and may be visible using precise optical/infrared imaging taken several years from now. A modest, all-sky survey telescope could detect ~10 such events per year, which would enable searches for very low-mass planetary companions to relatively nearby stars., Comment: 11 pages, 5 figures. Accepted to ApJ, to appear in the May 1, 2008 issue (v678). Minor changes. Data available upon request

We report on successes and failures in searching for positive superhumps in cataclysmic variables, and show the superhumping fraction as a function of orbital period. Basically, all short-period systems do, all long-period systems don't, and a 50% success rate is found at P_orb=3.1+-0.2 hr. We can use this to measure the critical mass ratio for the creation of superhumps. With a mass-radius relation appropriate for cataclysmic variables, and an assumed mean white-dwarf mass of 0.75 M_sol, we find a mass ratio q_crit=0.35+-0.02. We also report superhump studies of several stars of independently known mass ratio: OU Virginis, XZ Eridani, UU Aquarii, and KV UMa (= XTE J1118+480). The latter two are of special interest, because they represent the most extreme mass ratios for which accurate superhump measurements have been made. We use these to improve the epsilon(q) calibration, by which we can infer the elusive q from the easy-to-measure epsilon (the fractional period excess of P_superhump over P_orb). This relation allows mass and radius estimates for the secondary star in any CV showing superhumps. The consequent mass-radius law shows an apparent discontinuity in radius near 0.2 M_sol, as predicted by the disrupted magnetic braking model for the 2.1-2.7 hour period gap. This is effectively the "empirical main sequence" for CV secondaries., Comment: PDF, 45 pages, 9 tables, 12 figures; accepted, in press, to appear November 2005, PASP; more info at http://cba.phys.columbia.edu/

We report a photometric study of the WZ Sagittae-type dwarf nova PQ Andromedae. The light curve shows strong (0.05 mag full amplitude) signals with periods of 1263(1) and 634(1) s, and a likely double-humped signal with P=80.6(2) min. We interpret the first two as nonradial pulsation periods of the underlying white dwarf, and the last as the orbital period of the underlying binary. We estimate a distance of 150(50) pc from proper motions and the two standard candles available: the white dwarf and the dwarf-nova outburst. At this distance, the K magnitude implies that the secondary is probably fainter than any star on the main sequence -- indicating a mass below the Kumar limit at 0.075 M_sol. PQ And may be another "period bouncer", where evolution now drives the binary out to longer period., Comment: PDF, 13 pages, 2 figures; accepted, in press, to appear September 2005, PASP; more info at http://cba.phys.columbia.edu/

We report time-series photometry of the quiescent optical counterpart of the EUV transient RE J1255+266. The star appears as a DA white dwarf with bright emission lines and a complex spectrum of periodic signals in its light curve. A signal at 0.0829 d is likely to be the orbital period of the underlying cataclysmic binary (probably a dwarf nova). Characteristic periods of 1344, 1236, and 668 s are seen, and a host of weaker signals. We interpret these noncommensurate signals as (nonradial) pulsation periods of the white dwarf. The donor star is unseen at all wavelengths, and the accretion rate is very low. We estimate a distance of 180+-50 pc, and M_v=14.6+-1.3 for the accretion light. The binary probably represents a very late stage of evolution, with the donor star whittled down to M_2<0.05 M_sol. Such binaries stubbornly resist discovery due to their faintness and reluctance to erupt, but are probably a very common type of cataclysmic variable. If the signal at 0.0829 d is indeed the orbital period, then the binary is an excellent candidate as a "period bouncer". Plausible colleagues in this club include four dwarf novae and the (so far) noneruptive stars GD 552 and 1RXS J105010.3-140431 (hereafter RX 1050). The 1994 EUV eruption implies a soft X-ray/EUV luminosity of 10^(34-35) erg/s, greater than that of any other dwarf nova. We attribute that to a favorable blend of properties: a high-mass white dwarf; a very transparent line of sight (the "Local Chimney"); and a low binary inclination. The first maximizes the expected temperature and luminosity of boundary-layer emission; the other two increase the likelihood that soft X-rays can survive their perilous passage through an accretion-disk wind and the interstellar medium., Comment: PDF, 43 pages, 5 tables, 10 figures, 1 appendix; accepted, in press, to appear May 2005, PASP; more info at http://cba.phys.columbia.edu/

We report photometry of the helium-rich cataclysmic variable ES Ceti during 2001-2004. The star is roughly stable at V ~ 17.0 and has a light curve dominated by a single period of 620 s, which remains measurably constant over the 3 year baseline. The weight of evidence suggests that this is the true orbital period of the underlying binary, not a "superhump" as initially assumed. We report GALEX ultraviolet magnitudes, which establish a very blue flux distribution (F_nu ~ nu^1.3), and therefore a large bolometric correction. Other evidence (the very strong He II 4686 emission, and a ROSAT detection in soft X-rays) also indicates a strong EUV source, and comparison to helium-atmosphere models suggests a temperature of 130+-10 kK. For a distance of 350 pc, we estimate a luminosity of (0.8-1.7)x10^34 erg/s, yielding a mass accretion rate of (2-4)x10^-9 M_sol/yr onto an assumed 0.7 M_sol white dwarf. This appears to be about as expected for white dwarfs orbiting each other in a 10 minute binary, assuming that mass transfer is powered by gravitational radiation losses. We estimate mean accretion rates for other helium-rich cataclysmic variables, and find that they also follow the expected M-dot ~ P_o^-5 relation. There is some evidence (the lack of superhumps, and the small apparent size of the luminous region) that the mass transfer stream in ES Cet directly strikes the white dwarf, rather than circularizing to form an accretion disk., Comment: PDF, 26 pages, 3 tables, 9 figures; accepted, in press, to appear February 2005, PASP; more info at http://cba.phys.columbia.edu/

We report photometry and spectroscopy of the novalike variable DW Cancri. The spectra show the usual broad H and He emission lines, with an excitation and continuum slope characteristic of a moderately high accretion rate. A radial-velocity search yields strong detections at two periods, 86.1015(3) min and 38.58377(6) min. We interpret these as respectively the orbital period P_orb of the binary, and the spin period P_spin of a magnetic white dwarf. The light curve also shows the spin period, plus an additional strong signal at 69.9133(10) min, which coincides with the difference frequency 1/P_spin-1/P_orb. These periods are stable over the 1 year baseline of measurement. This triply-periodic structure mimics the behavior of several well-credentialed members of the "DQ Herculis" (intermediate polar) class of cataclysmic variables. DQ Her membership is also suggested by the mysteriously strong sideband signal (at nu_spin-nu_orb), attesting to a strong pulsed flux at X-ray/EUV/UV wavelengths. DW Cnc is a new member of this class, and would be an excellent target for extended observation at these wavelengths., Comment: PDF, 28 pages, 6 tables, 9 figures; accepted, in press, to appear June 2004, PASP; more info at http://cba.phys.columbia.edu/

We report the results of long observing campaigns on two novalike variables: V442 Ophiuchi and RX J1643.7+3402. These stars have high-excitation spectra, complex line profiles signifying mass loss at particular orbital phases, and similar orbital periods (respectively 0.12433 and 0.12056 d). They are well-credentialed members of the SW Sex class of cataclysmic variables. Their light curves are also quite complex. V442 Oph shows periodic signals with periods of 0.12090(8) and 4.37(15) days, and RX J1643.7+3402 shows similar signals at 0.11696(8) d and 4.05(12) d. We interpret these short and long periods respectively as a "negative superhump" and the wobble period of the accretion disk. The superhump could then possibly arise from the heating of the secondary (and structures fixed in the orbital frame) by inner-disk radiation, which reaches the secondary relatively unimpeded since the disk is not coplanar. At higher frequencies, both stars show another type of variability: quasi-periodic oscillations (QPOs) with a period near 1000 seconds. Underlying these strong signals of low stability may be weak signals of higher stability. Similar QPOs, and negative superhumps, are quite common features in SW Sex stars. Both can in principle be explained by ascribing strong magnetism to the white dwarf member of the binary; and we suggest that SW Sex stars are borderline AM Herculis binaries, usually drowned by a high accretion rate. This would provide an ancestor channel for AM Hers, whose origin is still mysterious., Comment: PDF, 41 pages, 4 tables, 16 figures; accepted, in press, to appear December 2002, PASP; more info at http://cba.phys.columbia.edu/

We present photometry and spectroscopy of HT Camelopardalis, a recently discovered X-ray-bright cataclysmic variable. The spectrum shows bright lines of H, He I, and He II, all moving with a period of 0.059712(1) d, which we interpret as the orbital period. The star's brightness varies with a strict period of 515.0592(2) s, and a mean full amplitude of 0.11 mag. These properties qualify it as a /bona fide/ DQ Herculis star (intermediate polar) -- in which the magnetism of the rapidly rotating white dwarf channels accretion flow to the surface. Normally at V=17.8, the star shows rare and very brief outbursts to V=12-13. We observed one in December 2001, and found that the 515 s pulse amplitude had increased by a factor of ~100 (in flux units). A transient orbital signal may also have appeared., Comment: PDF, 19 pages, 3 tables, 6 figures; accepted, in press, to appear June 2002, PASP; more info at http://cba.phys.columbia.edu/

We report photometry of 1RXS J232953.9+062814, a recently discovered dwarf nova with a remarkably short 64.2-minute orbital period. In quiescence, the star's light curve is that of a double sinusoid, arising from the "ellipsoidal" distortion of the Roche-lobe-filling secondary. During superoutburst, common superhumps develop with a period 3-4% longer than P_orb. This indicates a mass ratio M_2/M_1=0.19+-0.02, a surprisingly large value in so compact a binary. This implies that the secondary star has a density 2-3 times higher than that of other short-period dwarf novae, suggesting a secondary enriched by H-burning prior to the common-envelope phase of evolution. We estimate i=50+-5 deg, M_1=0.63 (+0.12, -0.09) M_sol, M_2=0.12 (+0.03, -0.02) M_sol, R_2=0.121 (+0.010, -0.007) R_sol, and a distance to the binary of 180+-40 pc., Comment: PDF, 17 pages, 3 tables, 5 figures; accepted, in press, to appear June 2002, PASP; more info at http://cba.phys.columbia.edu/

We present spectroscopy and time-series photometry of the newly discovered dwarf nova 1RXS J232953.9+062814. Photometry in superoutburst reveals a superhump with a period of 66.06(6) minutes. The low state spectrum shows Balmer and HeI emission on a blue continuum, and in addition shows a rich absorption spectrum of type K4 +- 2. The absorption velocity is modulated sinusoidally at P_orb = 64.176(5) min, with semi-amplitude K = 348(4) km/s. The low-state light curve is double-humped at this period, and phased as expected for ellipsoidal variations. The absorption strength does not vary appreciably around the orbit. The orbital period is shorter than any other cataclysmic variable save for a handful of helium-star systems and V485 Centauri (59 minutes). The secondary is much hotter than main sequence stars of similar mass, but is well-matched by helium-enriched models, indicating that the secondary evolved from a more massive progenitor. A preliminary calculation in which a 1.2 solar-mass star begins mass transfer near the end of H burning matches this system's characteristics remarkably well., Comment: accepted to Astrophysical Journal Letters; 14 pages, 3 eps figures + 1 jpg greyscale figure

We use the hot spot eclipse times of the newly discovered deeply-eclipsing dwarf nova IY UMa to trace out the shape of its disc during the late superhump era. We find an eccentric disc. We show that the brightness of the stream-disc impact region varies as expected with |deltaV|^2, where deltaV is the differential velocity of the stream with respect to the velocity of the disc at the impact point. We conclude that the hot spot is the source of late superhump light., Comment: 7 pages, 9 figures, accepted for publication in MNRAS. Minor changes to original content, additional modelling and figures

The eclipsing novalike cataclysmic variable star V348 Pup exhibits a persistent luminosity modulation with a period 6 per cent longer than its 2.44 hour orbital-period (Porb). This has been interpreted as a `positive superhump' resulting from a slowly precessing non-axisymmetric accretion disc gravitationally interacting with the secondary. We find a clear modulation of mid-eclipse times on the superhump period, which agrees well with the predictions of a simple precessing eccentric disc model. Our modelling shows that the disc light centre is on the far side of the disc from the donor star when the superhump reaches maximum light. This phasing suggests a link between superhumps in V348 Pup and late superhumps in SU UMa systems. Modelling of the full lightcurve and maximum entropy eclipse mapping both show that the disc emission is concentrated closer to the white dwarf at superhump maximum than at superhump minimum. We detect additional signals consistent with the beat periods between the implied disc precession period and both (1/2)Porb and (1/3)Porb., Comment: 13 pages, 13 figures, accepted for publication in MNRAS