NEW LIMITS ON THE ORBITAL PARAMETERS OF 1E 1048.1-5937 AND 1E 2259+586 FROM RXTE OBSERVATIONS

We report on two Rossi X-ray Timing Explorer (RXTE) observations of th e anomalous X-ray pulsars 1E 1048.1-5937 and 1E 2259+586. Both sources have continued their almost constant spin-down during 1995/96. We car ried out a search for orbital Doppler shifts, in their observed spin f requencies, deriving stringent limits on the projected semi-axis. Unle ss these systems have unlikely small inclinations, main-sequence compa nions can be excluded. If 1E 1048.1-5937 and 1E 2259+586 are indeed bi nary systems, their companion stars must be either white dwarfs, or he lium-burning stars with M less than or similar to 0.8 M-., possibly un derfilling their Roche lobe.