Long-term X-ray Timing of Ultra-compact Binaries

We propose short timing observations of V407 Vul, and RX J0806.3+2756; uniquebinary systems with the shortest inferred orbital periods known. Both systemsshow secular spin up of their phaselocked Xray and optical pulsations withrates consistent with orbital evolution driven by gravitational radiationlosses. Phase coherent timing of all current data for these sources isapproaching the level at which the second frequency derivative can be measured.The value of this quantity is an important discriminator for models of the Xrayemission in these objects (Dall.Osso et al. 2007; Deloye & Taam 2006), andfuture, precise constraints on this parameter require simply that the existingtiming baseline continue to be extended with short, annual observations. Here we propose such observations truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]