A new expectation function for fitting the OH curves of circumstellar maser lines.

For the purpose of measuring the periods and the spatial extent of OH masing regions in the circumstellar envelopes of OR/IR stars, it has become a common practice to use a piecewise sine function to model the OH flux curves. Although such a function offers a model which accounts for the asymmetry observed, we propose a new and simpler expectation function which reproduces the observed asymmetry naturally. This expectation function is also more reliable and easier to implement in a nonlinear least squares procedure. This model is applied to four cool variable stars which have been monitored for a period of roughly two years in the 1612 and 1667 MHz lines of the OH radical.