Spectroscopic binaries among AGB stars: The star V Hydrae, a binary eclipsed by its jet ?

Radial velocity (RV) monitoring is one of the methods to identify stars belonging to a binary system. However, asymptotic giant branch (AGB) stars in binary systems are more difficult to detect because their orbital signature is polluted by the regular envelope pulsation. The RV of a sample of suspected binary AGB stars has been monitored in order to detect their possible orbital motion. This RV monitoring has been performed using the high-resolution HERMES spectrograph installed on the Mercator telescope. 

Here we focus on the binary properties of the carbon star V Hydrae. The star is known to emit collimated bullets every 8.5 years, which is a common phenomenon during the transition from AGB to PN. It is known as well to emit a rapid jet (200 km/s), detected in the submm CO lines.

Our RV monitoring has shown that the orbital period is 17 years, which is just twice the recurrence time-scale of the bullet ejections, whereas the orbital period exactly matches the long-term modulation seen in the AAVSO light curve. The comparison of the RV and light curves reveals that the photometric obscuration (by several mags!) occurs at the time when the companion is in front of the AGB star, so that it is likely that these obscurations are caused by the jet emitted by (the accretion disc around?)  the companion. This situation is reminiscent of the one frequently occurring among post-AGB binary systems, so that V Hya thus appears to share properties of AGB and post-AGB systems.