A Hubble Space Telescope survey for novae in M87. III. Are novae good
  standard candles 15 days after maximum brightness?
  Ten weeks of daily imaging of the giant elliptical galaxy M87 with the Hubble
Space Telescope (HST) has yielded 41 nova light curves of unprecedented quality
for extragalactic cataclysmic variables. We have recently used these light
curves to demonstrate that the observational scatter in the so-called
Maximum-Magnitude Rate of Decline (MMRD) relation for classical novae is so
large as to render the nova-MMRD useless as a standard candle. Here we
demonstrate that a modified Buscombe - de Vaucouleurs hypothesis, namely that
novae with decline times t2 > 10 days converge to nearly the same absolute
magnitude about two weeks after maximum light in a giant elliptical galaxy, is
supported by our M87 nova data. For 13 novae with daily-sampled light curves,
well determined times of maximum light in both the F606W and F814W filters, and
decline times $t2 > 10 days we find that M87 novae display M(606W,15) = -6.37
+/- 0.46 and M(814W,15) = -6.11 +/- 0.43. If very fast novae with decline times
t2 < 10 days are excluded, the distances to novae in elliptical galaxies with
stellar binary populations similar to those of M87 should be determinable with
1 sigma accuracies of +/-20% with the above calibrations.
