We report new spectroscopic and photometric observations of the main-sequence , detached , eccentric , double-lined eclipsing binary V541 Cyg ( P = 15.34 days , e = 0.468 ) .
Using these observations together with existing measurements we determine the component masses and radii to better than 1 % precision : M _ { 1 } = 2.335 ^ { +0.017 } _ { -0.013 } ~ { } M _ { \sun } , M _ { 2 } = 2.260 ^ { +0.016 } _ { -0.013 } ~ { } M _ { \sun } , R _ { 1 } = 1.859 ^ { +0.012 } _ { -0.009 } ~ { } R _ { \sun } , and R _ { 2 } = 1.808 ^ { +0.015 } _ { -0.013 } ~ { } R _ { \sun } .
The nearly identical B9.5 stars have estimated temperatures of 10650 \pm 200 K and 10350 \pm 200 K. A comparison of these properties with current stellar evolution models shows excellent agreement at an age of about 190 Myr and { [ Fe / H ] } \approx - 0.18 .
Both components are found to be rotating at the pseudo-synchronous rate .
The system displays a slow periastron advance that is dominated by General Relativity ( GR ) , and has previously been claimed to be slower than predicted by theory .
Our new measurement , \dot { \omega } = 0.859 ^ { +0.042 } _ { -0.017 } deg century ^ { -1 } , has an 88 % contribution from GR and agrees with the expected rate within the uncertainties .
We also clarify the use of the gravity darkening coefficients in the light-curve fitting program EBOP , a version of which we use here .