New Symbiotic Stars Monitored with the Hermes Spectrograph: a Case Study of Orbital Hα Modulation

We present new orbital elements for two symbiotic systems involving a giant of spectral type S, namely V420 Hya and ER Del. These orbital elements are compared with existing elements for S-type binaries and for symbiotic binaries with M-giant primaries. It is shown that among the S-type binaries, most of the short-period systems (with P in the range 300 -- 1000 d) exhibit some kind of symbiotic activity (Hα emission, UV continuum, hard X-rays), but symbiotic systems are not restricted to the short-period systems. The complex and varying Hα profile of V420 Hya has been decomposed to several components (broad emission with σ ∼ 140 km/s, narrow emission with σ ∼ 60 km/s, and narrow absorption components). Their orbital modulation reveals that the broad emission is located close to the companion (under the hypothesis of a system with a mass ratio M_{g/M_c} = 2), and that this broad emission is mutilated by absorption from matter located along the line of sight and flowing towards the observer (faster than the giant) at all orbital phases.