510 Astrophysics, Vol. 57, No. 4, December , 2014 0571-7256/14/5704-0510 © 2014 Springer Science+Business Media New York Published in Astrofizika, Vol. 57, No. 4, pp. 553-562 (November 2014). Original article submitted September 19, 2014. INVESTIGATION OF FAINT GALACTIC CARBON STARS FROM THE FIRST BYURAKAN SPECTRAL SKY SURVEY. OPTICAL VARIABILITY. I. N-TYPE AGB CARBON STARS. K-BAND ABSOLUTE MAGNITUDES AND DISTANCES K. S. Gigoyan 1 , A. Sarkissian 2 , D. Russeil 3 , N. Mauron 4 , G. Kostandyan 1 , R. Vartanian 5 , H. V. Abrahamyan 1 , and G. M. Paronyan 1 The goal of this paper is to present an optical variability study of the comparatively faint carbon (C) stars which have been discovered by searching the First Byurakan Survey (FBS) low-resolution (lr) spectral plates at high Galactic latitudes using recent wide-area variability databases. The light curves from the Catalina Sky Survey (CSS) and Northern Sky Variability Survey (NSVS) databases were ex- ploited to study theit variability nature. In this paper, first in this series, the variability classes are presented for 54 N-type Asymptotic Giant Branch (AGB) C stars. One finds that 9 stars belongs to the group of Mira-type, 43 are Semi-Regular (SR), and 2 stars are Irregular (Irr)-type variables. The variabil- ity types of 27 objects has been established for the first time. K-band absolute magnitudes, distances, and height from the Galactic plane were estimated for all of them. We aim to better understand the nature of the selected C stars through spectroscopy, 2MASS photometric colors, and variability data. Most of the tools used in this study are developed within the framework of the Astronomical Virtual Observatory. Keywords: stars: N-type AGB stars: variability: K-band luminosity: distances 1 V.A.Ambartsumian Byurakan Astrophysical Observatory, Armenia, e-mail: kgigoyan@bao.sci.am 2 Universite de Versailles Saint-Quentin, CNRS/INSU, LATMOS-IPSL, France, e-mail: Alain.Sarkissian@latmos.ipsl.fr 3 Laboratoire d'Astrophysique de Marseille, CNRS-AMU, France, e-mail: delphine.russeil@lam.fr 4 Laboratoire Univers et Particules de Montpellier, UMR 5299 CNRS & Universite Montpellier II, France, e-mail: Nicolas.Mauron@univ-montp2.fr 5 LA Testing, 520 Mission Street South Pasadena, CA 91030, USA, e-mail: rafikvartanian@yahoo.com DOI 10.1007/s10511-014-9354-5