3 Observation and data analysis

Spectroscopic observations were undertaken on the 2.16 m optical telescope at the Xing-Long station of the Beijing Astronomical Observatory (BAO). The OMR (Optomechanics Research Inc.) spectrograph and the detector TEK1024 CCD were used in both runs of observations. The low-resolution spectroscopy (with dispersion of 400 Å/mm, 9.6- Å per pixel) centered at 6500 Å has been carried out on 4 observing nights from 1996 Nov. 28 to Dec. 1 under good sky conditions, whereas the intermediate-resolution spectroscopic observation (with dispersion of 50 Å/mm, 1.2- Å per pixel) centered at 6300 Å has been performed on 1996 Dec. 22, 23, 27 and 28, with usually two spectrophotometric standard stars observed each night for flux calibration. Furthermore, a grid of spectral standards was acquired during the second run by observing stars of spectral types F5 to M0 and of luminosity classes IV and V, which were selected from the Bright Star Catalog (1991).

All the observational spectral data were reduced with standard procedures in the NOAO Interactive Reduction and Analysis Facility (IRAF, version 2.10.4) software packages, and a relative flux calibration of each spectrum was performed using a mean response function. The low-resolution spectra were used to classify spectral types and to derive V₅₅₇₀. The intermediate-resolution spectra were normalized for further study. A custom routine programmed with the command language (CL) of IRAF, namely ASCLTS (Automatic Spectral Classification of Late Type Stars, Li & Hu 1997) was applied to determine the spectral types of the late type stars, the accuracy of which is expected to be ${\pm}$ one sub-class in most cases. Cross-correlation of each intermediate-resolution spectrum with the grid of spectral standards was carried out with ASCLTS to obtain the best fit spectral type. Then, the residual spectrum derived from the target spectrum and the standard spectrum of the same spectral type was used to calculate the equivalent width of H$_{\alpha}$ and LiI absorption with corresponding tasks and commands in IRAF.