Constraining Brans-dicke Gravity with Accreting Millisecond Pulsars in Ultracompact Binaries

The five accreting, millisecond X-ray pulsars in ultracompact binaries that were recently discovered with the Rossi X-ray Timing Explorer provide excellent candidates for constraining the deviations from general relativity described by the Brans-Dicke parameter ωBD. I calculate the expected rate of change of the orbital periods of these binaries and discuss the prospects of constraining ωBD to values that are an order of magnitude larger than current constraints. Finally, I show how measurements of the orbital period derivative in ultracompact binaries can be used to place lower bounds on their orbital inclination. Subject headings: gravitation–X-rays: binaries