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Singly-ionized ytterbium, with ultra-narrow optical clock transitions at 467nm and 436 nm, is a convenient system for the realization of optical atomicclocks and tests of present-day variation of fundamental constants. We presentthe first direct measurement of the frequency ratio of these two clocktransitions, without reference to a cesium primary standard, and using the samesingle ion of 171Yb+. The absolute frequencies of both transitions are alsopresented, each with a relative standard uncertainty of $6\times 10^{-16}$.Combining our results with those from other experiments, we report a three-foldimprovement in the constraint on the time-variation of the proton-to-electronmass ratio, $\dot{\mu}/{\mu} = 0.2(1.1)\times 10^{-16}$ year$^{-1}$, along withan improved constraint on time-variation of the fine structure constant,$\dot{\alpha}/{\alpha} = -0.7(2.1)\times 10^{-17}$ year$^{-1}$.

Frequency ratio of two optical clock transitions in $^{171}$Yb$^+$ and constraints on the time-variation of fundamental constants