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We present extensive tables of bolometric corrections and interstellar extinction coefficients for the WFPC2 and ACS (both WFC and HRC) photometric systems. They are derived from synthetic photometry applied to a database of spectral energy distributions covering a large range of effective temperatures, surface gravity, and metal content. Carbon stars are also considered. The zero points take into consideration the new high-accuracy Vega fluxes from Bohlin. These tables are employed to transform Padova isochrones into WFPC2 and ACS photometric systems using interstellar extinction coefficients on a star-to-star basis. All data are available either in tabular form or via an interactive web interface in the case of the isochrones. Preliminary tables for the WFC3 camera are also included in the database.

The theoretical prediction that trigonometric parallaxes suffer from a statistical effect has become topical again now that the results of the Hipparcossatellite have become available. This statistical effect, the so‐called Lutz–Kelker bias, causes observed parallaxes to be too large. This has the implication that inferred distances, and hence inferred luminosities are too small. Published analytic calculations of the Lutz–Kelker bias indicate that the inferred luminosity of an object is, on average, 30 per cent too small when the error in the parallax is only 17.5 per cent. Yet, this bias has never been determined empirically. In this paper we investigate whether there is such a bias by comparing ground‐based measurements with the best Hipparcosparallaxes. We find that there is indeed a large bias with an average and scatter comparable to predictions. We propose a simple method to correct for the LK bias, and apply it successfully to a subsample of our stars. We then analyse the sample of the 26 ‘best’ Cepheids used by Feast & Catchpole to derive the zero‐point of the period–luminosity relation. The final result is based on the 20 fundamental mode pulsators and leads to a distance modulus to the Large Magellanic Cloud — based on Cepheid parallaxes — of 18.56 ± 0.08, consistent with previous estimates.

The I‐band brightness MIof clump stars is a possible distance indicator for stellar populations. Investigations have shown that MIis almost insensitive to the V − Icolour within the clump. Based on this, it was assumed that MIwas insensitive to age and composition of the stellar population and therefore an ideal standard candle, which could be calibrated with local clump stars whose absolute brightness is known from Hipparcos parallaxes. This resulted in a distance to the Large Magellanic Cloud about 15 per cent smaller than usually determined.  In the present paper we show that with a population synthesis approach we can reproduce the constancy of MIwith colour for the local Hipparcosclump sample. Nevertheless, MIis not a constant among different populations, but depends on metallicity. As a result, the calculated distance modulus to the LMC of 18.28 ± 0.14 mag is in better agreement with standard values. This resolves, at least partially, the controversial result obtained by the assumption of a universal value for MI.  Particularly remarkable is our prediction that stars slightly heavier than the maximum mass for developing degenerate He cores, MHef, should define a secondary clumpy structure, about 0.3 mag below the bluest extremity of the red clump. Both features are well separated in the MIversus V − Idiagram of metal‐rich stellar populations. Indeed, this secondary clump can be clearly identified in the Hipparcosdata base of stars with reliable Iphotometry and parallax errors smaller than 10 per cent. Since the stars in this feature should represent a narrow range of masses, their mass determination, e.g. by the use of binary systems, can provide information about the efficiency of convective overshooting from stellar cores.  Our investigation demonstrates that the red giant branch clump cannot be used as a distance indicator without proper knowledge and modelling of the population under investigation. In addition, there remain unsolved problems in the models, such as correct bolometric corrections and colour transformations.