  We present new data on Cherenkov light observations obtained during the
period 1994-2009, after a modernization of the Yakutsk EAS array. A complex
analysis of $\xmax$ and its fluctuations $\sigma(\xmax)$ was performed over a
wide energy range. With the new data, according to QGSJet II model, an
estimation was made of the cosmic ray mass composition for $\E \sim 10^{17} - 3
\times 10^{19}$ eV. The result points towards a mixed composition with a large
portion of heavy nuclei at $\E \sim 10^{17}$ eV and the dominance of light
nuclei at $\E \sim 10^{19}$ eV. The analysis of $\sigma(\xmax)$ energy
dependence for the same energies qualitatively confirms this result. The shape
of the $\xmax$ distribution at fixed energy $10^{18}$ eV is analysed to make
more precise conclusions on cosmic ray mass composition.
