The Long-term Monitoring of the Extreme Galactic Cepheids V810 Centauri and V473 Lyrae

Two peculiar galactic supergiants have been monitored in photometry and radial velocity during more than 15 years, using the telescopes of Geneva Observatory at La Silla (ESO, Chile) and Haute-Provence Observatory (France). These two supergiants are found to be the classical cepheids with the longest (V810 Cen) and shortest (V473 Lyr) periods in our Galaxy, i.e. 153 d and 1.5 d. In addition, V810 Cen is a double-mode cepheid with a period ratio of 0.68, strongly suggesting pulsation in the fundamental mode and rst overtone, and V473 Lyr exhibits large variations of amplitude in a time scale larger than 1000 d. The understanding of the complex variability of these two stars requires long-term{, continuous{ and high-precision monitorings.