How is the orbital period of a test particle modified by the Dvali–Gabadadze–Porrati gravity?

In addition to the pericentre, the mean anomaly M and, thus, the mean longitude, also the orbital period Pb and the mean motion n of a test particle are modified by the Dvali-Gabadadze-Porrati gravity. While the correction to Pb depends on the mass of the central body and on the geometrical features of the orbital motion around it, the correction to n is independent of them, up to terms of second order in the eccentricity e. The latter one amounts to about 2 10^-3 arcseconds per century. The present-day accuracy in determining the mean motions of the inner planets of the Solar System from radar ranging and differential Very Long Baseline Interferometry is 10^-2-5 10^-3 arcseconds per century, but it should be improved in the near future when the data from the spacecraft to Mercury and Venus will be available.