We have used an optical frequency comb referenced to coordinated universal time ͑UTC͒ to perform spectroscopic measurements of the D 1 transition in laser-cooled 85 Rb and 87 Rb. Our measurements of the optical frequencies have uncertainties of 28 kHz for 85 Rb and 79 kHz for 87 Rb. These measurements were used to calculate the magnetic dipole constant A͑ 2 P 1/2 ͒ for 85 Rb and 87 Rb with similar ranges of uncertainty. Previously, A͑ 2 P 1/2 ͒ for these isotopes was determined with a stated

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... 5 kHz uncertainty level; however, there is a large discrepancy, 140 kHz for 85 Rb and 2.2 MHz for 87 Rb, between the two apparently most precise measurements. Our transition frequency measurements, which avoid saturated absorption spectroscopy and transfer resonator systematics, help resolve this disagreement.