Infrared spectroscopy of symbiotic stars. I. Orbits for well-known S-type systems

First results are reported for a program of monitoring symbiotic-star veloc ities in the 1.6 mu m region with infrared-array technology. Infrared radia l velocities have been used to determine single-lined spectroscopic orbits for six well-known symbiotic stars, EG And, T CrB, CI Cyg, EX Mon, RS Oph, and AG Peg. The new orbits are in general agreement with previous orbits de rived from optical velocities. From the combined optical and infrared veloc ities improved orbital elements for the six systems have been determined. E ach of the orbital periods has been determined solely from the radial-veloc ity data. With the addition of our new velocities, the orbital period of EX Mon has been revised to 1259 days, a 10% decrease from the previously repo rted result.