The Henize sample of S stars. IV. New symbiotic stars

The properties of the few symbiotic stars detected among the 66 binary S stars from the Henize sample are discussed. Two stars (Hen 18 and Hen 121) exhibit both a strong blue-violet continuum and strong H_alpha emission (FWHM of 70 km s^-1), whereas Hen 134 and 137 exhibit weak H_alpha emission. The H_alpha profiles are typical of non-dusty symbiotic stars belonging to class S-3 as defined by Van Winckel et al. (1993, A&AS 102, 401). In that class as in the Henize symbiotic S stars, He I, [N II] or [S II] emission lines are absent, suggesting that the nebular density is high but the excitation rather low. The radial velocity of the centre of the H_alpha emission is identical to that of the companion star (at least for Hen 121 where this can be checked from the available orbital elements), thus suggesting that the H_alpha emission originates in gas moving with the companion star. For Hen 121, this is further confirmed by the disappearance of the ultraviolet Balmer continuum when the companion is eclipsed by the S star. Hen 121 is thus the second eclipsing binary star discovered among extrinsic S stars (the first one is HD 35155). A comparison of the available data on orbital periods and H_alpha emission leads to the conclusion that H_alpha emission in s stars seems to be restricted to binary systems with periods in the range 600-1000 d, in agreement with the situation prevailing for red symbiotic stars (excluding symbiotic novae). Symbiotic S stars are found among the most evolved extrinsic S stars. Based on observations carried out at the European Southern Observatory (ESO, La Silla, Chile; program 60.E-0805) and at the Swiss 70 cm telescope (La Silla, Chile).