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The accreting millisecond pulsars IGR J00291+5934 and SAX J1808.4-3658 aretwo compact binaries with very similar orbital parameters. The latter has beenobserved to evolve on a very short timescale of ~70 Myr which is more than anorder of magnitude shorter than expected. There is an ongoing debate on thepossibility that the pulsar spin-down power ablates the companion generatinglarge amount of mass-loss in the system. It is interesting therefore to studywhether IGR J00291+5934 does show a similar behaviour as its twin system SAXJ1808.4-3658. In this work we present the first measurement of the orbitalperiod derivative of IGR J00291+5934. By using XMM-Newton data recorded duringthe 2015 outburst and adding the previous results of the 2004 and 2008outbursts, we are able to measure a 90% confidence level upper limit for theorbital period derivative of -5x10^-13<Pb_dot<6x10^-13. This implies that thebinary is evolving on a timescale longer than ~0.5 Gyr, which is compatiblewith the expected timescale of mass transfer driven by angular momentum lossvia gravitational radiation. We discuss the scenario in which the power lossfrom magnetic dipole radiation of the neutron star is hitting the companionstar. If this model is applied to SAX J1808.4-3658 then the difference inorbital behavior can be ascribed to a different efficiency for the conversionof the spin-down power into energetic relativistic pulsar wind andX-ray/gamma-ray radiation for the two pulsars, with IGR J00291+5934 requiringan extraordinarily low efficiency of less than 5% to explain the observations.Alternatively, the donor in IGR J00291+5934 is weakly/not magnetized whichwould suppress the possibility of generating mass-quadrupole variations.

The Slow Orbital Evolution of the Accreting Millisecond Pulsar IGR J0029+5934