  Aims. To obtain limits on the variation of the fine-structure constant alpha
and the electron-to-proton mass ratio mu over different cosmological epochs.
Methods. A new approach based on the comparison of redshifts of far infrared
(FIR) fine-structure lines and low-lying rotational transitions in CO is
proposed which is in principle more sensitive by a factor of 10 compared to QSO
metal absorption lines. Results. Estimations of the quotient F = alpha^2/mu
obtained for two distant quasars J1148+5251 (z = 6.42) and BR1202-0725 (z =
4.69) provide Delta F/F = (0.1+/-1.0)10^{-4} and (1.4+/-1.5)10^{-4},
respectively. The obtained limits are consistent with no variation of physical
constants at the level of 0.01% over a period of 13 Gyr. Conclusions. Upcoming
observations of quasars and distant galaxies in FIR fine-structure lines of
different species and in CO low rotational lines at the SOFIA, HSO, and ALMA
are expected to improve the current limit by, at least, an order of magnitude.

Astrophysics;Atomic Physics