Photometry of V407 Vul and RXJ0806.3+1527: Twin systems ?

Abstract

We present new photometry of V407 Vul and RXJ0806.3+1527. These stars are thought to be ultra-compact pairs of white dwarfs and the shortest period binaries known. They have periods of 569 and 321 sec respectively. We found that the X-ray and optical light curves of RXJ0806.3+1527 are not anti-phased as was previously thought. Instead, the X-ray lags the optical by 0.2 of cycle, indistinguishable from the phase offset seen in V407 Vul. Although very similar to RXJ0806.3+1527, V407 Vul surprisingly is observed to have the spectrum of a G star. We test whether this could be a line-of-sight coincidence by searching for variations in the V407 Vul's position on the sky on the 569 second period. We find that the G star is separated from the variable by 0.03 arcsec, and has no direct connection with the 569 second source. This supports a common nature for V407 Vul and RXJ0806.3+1527.