Improved constraints on possible variation of physical constants from HI 21-cm and molecular QSO absorption lines

Quasar (QSO) absorption spectra provide an extremely useful probe of possib le cosmological variation in various physical constants. Comparison of H I 21-cm absorption with corresponding molecular (rotational) absorption spect ra allows us to constrain variation in y = alpha (2) g(p), where alpha is t he fine-structure constant and g(p) is the proton g-factor. We analyse spec tra of two QSOs, PKS 1413+135 and TXS 0218+357, and derive values of Deltay /y at absorption redshifts of z = 0.2467 and 0.6847 by simultaneous fitting of the H I 21-cm. and molecular lines. We find Deltay/y = (-0.20 +/- 0.44) x 10(-5) and Deltay/y = (-0.16 +/- 0.54) x 10(-5) respectively, indicating an insignificantly smaller y in the past. We compare our results with othe r constraints from the same two QSOs given recently by Drinkwater et al. an d Carilli et al., and with our recent optical constraints, which indicated a smaller a at higher redshifts.