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We have built an infrared beam combiner for the GI2T/REGAIN interferometer ofthe Observatoire de la Cote d'Azur. The beam combiner allows us to recordspectrally dispersed Michelson interference fringes in the near-infrared J-, H-or K-bands. The beam combiner has the advantage that Michelson interferogramscan simultaneously be recorded in about 128 different spectral channels. Thetilt of the spectrally dispersed fringes is a measure of the instantaneousoptical path difference. We present the optical design of the beam combiner andGI2T/REGAIN observations of the Mira star R Cas with this beam combiner in thespectral range of 2.00 micron - 2.18 micron (observations on 22 and 25 August1999; variability phase 0.08; V-magnitude approx. 6; seven baselines between12m and 24m; reference stars Vega and Beta Peg). The spectrograph of the beamcombiner consists of an anamorphotic cylindrical lens system, an image planeslit, and a grism. A system of digital signal processors calculates theensemble average power spectrum of the spectrally dispersed Michelsoninterferograms and the instantaneous optical path difference error in realtime. From the observed R Cas visibilities at baselines 12.0m, 13.8m and 13.9m,a 2.1 micron uniform-disk diameter of 25.3mas +/-3.3mas was derived. Theunusually high visibility values at baselines >16m show that the stellarsurface of R Cas is more complex than previously assumed. The visibility valuesat baselines >16m can be explained by high-contrast surface structure on thestellar surface of R Cas or other types of unexpected center-to-limbvariations. The R Cas observations were compared with theoretical Mira starmodels yielding a linear Rosseland radius of 276Rsun +/-66Rsun and an effectivetemperature of 2685K+/-238K for R Cas at phase 0.08.

GI2T/REGAIN spectro-interferometry with a new infrared beam combiner