We present a ground based photometry of the low-temperature contact binary BB Peg .
We collected all times of mid-eclipses available in literature and combined them with those obtained in this study .
Analyses of the data indicate a period increase of 3.0 ( 1 ) \times 10 ^ { -8 } days/yr .
This period increase of BB Peg can be interpreted in terms of the mass transfer 2.4 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } from the less massive to the more massive component .
The physical parameters have been determined as M _ { \mathrm { c } } = 1.42 M _ { \odot } , M _ { \mathrm { h } } = 0.53 M _ { \odot } , R _ { \mathrm { c } } = 1.29 R _ { \odot } , R _ { \mathrm { h } } = 0.83 R _ { \odot } , L _ { \mathrm { c } } = 1.86 L _ { \odot } , and L _ { \mathrm { h } } = 0.94 L _ { \odot } through simultaneous solution of light and of the radial velocity curves .
The orbital parameters of the third body , that orbits the contact system in an eccentric orbit , were obtained from the period variation analysis .
The system is compared to the similar binaries in the Hertzsprung-Russell and Mass-Radius diagram .