Optical interferometric measurements of gamma Cassiopeiae's envelope in the H-alpha line.

We report observations of ΥCas in the continuum and in the Hα emission line, with the CERGA small interferometer I2T, providing angular resolution down to the milli-arcsecond level. We used a differential method, described by Labeyrie (1982), which measures the modulus and the phase of the object visibility function, in multiple spectral channels. We obtained a 5.2 milli-arcsecond angular diameter for the envelope, and found that the envelope was centered on the star with its red and blue wings symmetrically positioned south and north.