A Hubble Space Telescope survey for novae in M87-III. Are novae good standard candles 15 d 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 d 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 d 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 d are excluded, the distances to novae in elliptical galaxies with stellar binary populations similar to those of M87 should be determinable with 1s accuracies of ± 20 per cent with the above calibrations.