Multiwavelength optical observations of chromospherically active binary systems. 1. Simultaneous H alpha, Na I D_1, D_2, and He I D_3 observations

This is the first paper of a series aimed at studying the chromosphere of active binary systems using the information provided for several optical spectroscopic features. Simultaneous H_α, Na I D_1, D_2, and He I D_3 spectroscopic observations are reported here for 18 systems. The chromospheric contribution in these Lines have been determined using the spectral subtraction technique. Very broad wings have been found in the subtracted H_α profile of some of the more active stars. These profiles are well matched using a two-components Gaussian fit (narrow and broad) and the broad component could be interpreted as arising from microflaring. Prominence-like extended material have been detected in a near-eclipse H_α observation of the system AR Lac. The excess emission found in the Na I D_1 and D_2 lines by application of the spectral subtraction technique and the behaviour of the H_α line in the corresponding simultaneous observations indicate that the filling-in of the core of these lines is a chromospheric activity indicator. For giant stars of the sample the He I D_3 line has been detected in absorption in the subtracted spectra. An optical flare has been detected in UX Ari and II Peg through the presence of the He I D_3 in emission incoincidence with the enhancement of the H_α emission