CALIBRATION OF NOVAE AS DISTANCE INDICATORS

The problem of the correct calibration of the maximum magnitude versus rate of decline relationship for Galactic novae is discussed, on the basis of the properties of the nova populations of M31 and the Large Magellanic Cloud (LMC). We have compared the ''observed'' maximum magnitude versus rate of decline (hereafter MMRD) distributions of M31 and LMC novae with the most widely used analytic representations of the Galactic MMRD relationship, and we find that the linear regressions, normally used to interpolate the Galactic data, fit the M31 and LMC distributions only as very rough, first-order approximations. We have used the bright and fast objects belonging to the LMC nova population to improve the fit of the bright end of the Galactic MMRD relationship, previously based only on two fast novae of M31. The new results indicate that at least two O-Ne-Mg novae, Nova Cyg 1992 and Nova Her 1991, do not deviate, within the observational errors, from the main tract of the improved relationship. The new calibration, applied to novae discovered in Virgo, gives a distance to the cluster of 18.6 +/- 3.3 Mpc.