Orbital motion of the secondary in three AM Her systems: evidence for low-mass white dwarfs

In an earlier paper (Mukai & Charles 1986), we have reported the spectroscopic detection of the secondaries in the three AM Her type systems, CW1103+254, PG1550+191 and E2003+225. In this paper, we present our high-resolution spectrophotometry of these stars in the wavelength region λλ7550–8850. In all three cases, we have successfully detected the radial velocity variation of the second star using the Na I doublet at λλ8183-94. Combined with values for each system’s inclination estimated from polarimetry, we derive constraints on the white dwarf masses. The average of our best estimates for the white dwarf masses in these systems and AM Her itself is $$\sim0.6\,{M}_{\odot}$$, essentially coincident with the masses of isolated white dwarfs.