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 possible cosmological variation in various physical constants. Comparison of H I 21-cm absorption with corresponding molecular (rotational) absorption spectra allows us to constrain variation in y = alpha (2) g(p), where alpha is the fine-structure constant and g(p) is the proton g-factor. We analyse spectra 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 other constraints from the same two QSOs given recently by Drinkwater et al. and Carilli et al., and with our recent optical constraints, which indicated a smaller a at higher redshifts.