3 Method of analysis

The variability curves presented here deal with the integrated fluxes of the red, blue and intermediate peaks, when they exist, in each emitting OH maser line as well as the maximum intensity variations of the spectral components. These latter components were determined through a spectral decomposition by Gaussian fitting (Etoka 1996).

Observations of Miras at high velocity resolution, show that the spectral profile of these sources is not smooth, but is in fact composed of numerous rather narrow components blended together. Up to now, only the global integrated flux has been considered. According to a spectral decomposition method based on Gaussian fitting (Etoka 1996), we are able to study the OH variations in a more precise way. The accepted tolerance on the velocity divergence under which a component was considered to be the same from one fit to another was taken to be 1.5 times the poorest velocity resolution of the whole data set (i.e., 0.14 km s^-1). This leads to a tolerated divergence of 0.2 km s^-1.

Let us define the degree of circular polarization as follows:

$$% {\rm [RHC-LHC]_{freq.} = \frac{Flux_{RHC} - Flux_{LHC}} {Flux_{RHC} + Flux_{LHC}} }$$ (1)

where the flux can be the integrated or the maximum intensity flux depending on the case, and where "LHC'' and "RHC'' are respectively the left- and right-handed polarizations. The subscript "freq.'' refers to the frequency for which the degree of polarization has been calculated (i.e., 1612, 1665 or 1667 MHz).
For the calculation of the asymmetry factors we have used the curve fitting program developed by David et al. (1996) on the best sampled OH curve of integrated flux of each star over, when possible, 2 consecutive cycles.