Ture of Long Secondary Periods in Agb Stars

We obtained BV IC photometry of IRC−10443 in 85 different nights distributed over two years, and in addition low resolution absolute spectrophotometry and high resolution Echelle spectroscopy. Our data show that IRC −10443, which was never studied before in any detail, is a SRa variable, characterized by ∆B=1.27, ∆V=1.14 and ∆I=0.70 mag amplitudes and mean values =13.75, =11.33 and =6.18 mag. Two strong periodicities are simultaneously present: a principal one of 85.5 (±0.2) days, and a secondary one of 620 (±15) days, both sinusoidal in shape, and with semi-amplitudes ∆V=0.41 and 0.20 mag, respectively. IRC−10443 turns out to be a M7III star, with a mean heliocentric radial velocity−28 km/s and reddened by EB−V =0.87, a third of which of circumstellar origin. The same 0.5 kpc distance is derived from application of the appropriate period-luminosity relations to both the principal and the secondary periods. The long secondary period causes a sinusoidal variation in color of 0.13 mag semi-amplitude in V −IC, with IRC−10443 being bluest at maximum and reddest at minimum, and with associated changes in effective temperature and radius of 85 K and 6%, respectively. This behavior of colors argues in favor of a pulsation nature for the still mysterious long secondary periods in AGB stars.