  Aims. We constrain a hypothetical variation in the fundamental physical
constants over the course of cosmic time. Methods. We use unique observations
of the CO(7-6) rotational line and the [CI] 3P_2 - 3P_1 fine-structure line
towards a lensed galaxy at redshift z = 5.2 to constrain temporal variations in
the constant F = alpha^2/mu, where mu is the electron-to-proton mass ratio and
alpha is the fine-structure constant. The relative change in F between z = 0
and z = 5.2, dFF = (F_obs - F_lab)/F_lab, is estimated from the radial velocity
offset, dV = V_rot - V_fs, between the rotational transitions in carbon
monoxide and the fine-structure transition in atomic carbon. Results. We find a
conservative value dV = 1 +/- 5 km/s (1sigma C.L.), which when interpreted in
terms of dFF gives dFF < 2x10^-5. Independent methods restrict the
mu-variations at the level of dmm < 1x10^-7 at z = 0.7 (look-back time t_z0.7 =
6.4 Gyr). Assuming that temporal variations in mu, if any, are linear, this
leads to an upper limit on dmm < 2x10^-7 at z = 5.2 (t_z5.2 = 12.9 Gyr). From
both constraints on dFF and dmm, one obtains for the relative change in alpha
the estimate daa < 8x10^-6, which is at present the tightest limit on daa at
early cosmological epochs.
