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Ultraluminous X-ray sources openParULXsclosePar are X-ray binaries radiating above the Eddington luminosity. An increasing sample of ULXs are known to be powered by accreting pulsars openParPULXsclosePar comma and it is currently unclear if their observed emission is amplified by strong radiation beaming or it denotes a genuine super-Eddington mass accretion rate. In the second case comma the large mass transfer required is expected to produce detectable effects on the binary orbit comma which were tentatively detected in past observations. In this program comma we use pulsar timing to track the orbital evolution of the first discovered PULX comma M82 X-2 comma producing a strong constraint on the mass transfer and finally discriminate between the beaming and the extreme super-Eddington models.