2007ApJ...666.1174R

We present absolute parallaxes and relative proper motions for five AM CVn stars, which we obtained using the Fine Guidance Sensors on board the Hubble Space Telescope. Our parallax measurements translate into distances d_AMCVn=606⁺¹³⁵_–93pc, d_HPLib=197⁺¹⁴_–12pc, d_CRBoo=337⁺⁴⁴_–35 pc, d_V803Cen=347⁺³²_–27pc, and d_GPCom=75⁺²_–2pc. From these distances we estimate the space density of AM CVn stars and suggest that previous estimates have been too high by about an order of magnitude. We also infer the mass accretion rates, which allows us to constrain the masses of the donor stars, and we show that relatively massive, semidegenerate donor stars are favored in all systems except GP Com. Finally, we give updated estimates for the systems' gravitational wave signals, based on their distances and the inferred masses of the binary components, which are relevant for future space missions such as the proposed Laser Interferometer Space Antenna (LISA). We show that all systems but GP Com are excellent candidates for detection with LISA.