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It is commonly believed that millisecond radio pulsars have been spun up bytransfer of matter and angular momentum from a low-mass companion during anX-ray active mass transfer phase. A subclass of low-mass X-ray binaries is thatof the accreting millisecond X-ray pulsars, transient systems that show periodsof X-ray quiescence during which radio emission could switch on. The aim ofthis work is to search for millisecond pulsations from three accretingmillisecond X-ray pulsars, XTE J1751-305, XTE J1814-338, and SAX J1808.4-3658,observed during their quiescent X-ray phases at high radio frequencies (5 - 8GHz) in order to overcome the problem of the free-free absorption due to thematter engulfing the system. A positive result would provide definite proof ofthe recycling model, providing the direct link between the progenitors andtheir evolutionary products. The data analysis methodology has been chosen onthe basis of the precise knowledge of orbital and spin parameters from X-rayobservations. It is subdivided in three steps: we corrected the time series forthe effects of (I) the dispersion due to interstellar medium and (II) of theorbital motions, and finally (III) folded modulo the spin period to increasethe signal-to-noise ratio. No radio signal with spin and orbitalcharacteristics matching those of the X-ray sources has been found in oursearch, down to very low flux density upper limits. We analysed severalmechanisms that could have prevented the detection of the signal, concludingthat the low luminosity of the sources and the geometric factor are the mostlikely reasons for this negative result.

Search for pulsations at high radio frequencies from accreting millisecond X-ray pulsars in quiescence