IRAS 08544-4431: A new post-AGB star in a binary system surrounded by a dusty disc

We present an analysis of our extensive data-set on IRAS 08544-4431. It is the first object we discuss of our newly defined sample of stars, selected for their position in the "RV Tauri" box in the IRAS [12]-[25], [25]-[60] two-color diagram. Moreover, our selection criteria included an observed excess in the L-band, indicative of a dusty disc. The SED of IRAS 08544-4431 shows a broad IR excess starting already at H. Our optical photometric data reveal some evidence for deep and shallow minima in the light curve and a pulsation time-scale of around 100 days with a small amplitude ( (delta)Vpeak-to-peak = 0.17 mag). Our CORALIE radial velocity measurements show that IRAS 08544-4431 is a binary system with a period of 499 (plus-minus) 3 days and a mass function of 0.02 M. Moreover, IRAS 08544-4431 is detected in both the CO (2-1) and (1-0) mm-wave emission lines. The triangular shape of the weak CO profile confirms that part of the circumstellar material is not freely expanding but resides probably in a dusty circumbinary disc. Our chemical abundance analysis of a high resolution spectrum of high S/N reveals that a depletion process has modified the photospheric abundances to a moderate extent ( [Zn/Fe]=+0.4). All these findings confirm that the F-type IRAS 08544-4431 is another good example of a binary Post-AGB star surrounded by a dusty disc. The H(alpha) P-Cygni profile shows ongoing mass-loss with a very high outflow velocity, the origin of which is not understood. The strength and velocity of the H (alpha)-absorption are modulated with the orbital motion; the maxima of both quantities (~400 , 5 A* respectively) occur at superior conjunction.