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We present the first long-term study of the optical counterpart to the X-raypulsar LS V +44 17/RX J0440.9+4431. The results of our photometric andspectroscopic analysis show that LS V +44 17/RX J0440.9+4431 contains amoderately reddened, E(B-V)=0.65+-0.05, B0.2V star located at about 3.3 kpc.The Ha line consistently shows a double-peak profile varying from symmetricshape to completely distorted on one side (V/R phases). A correlation betweenthe equivalent width of the Ha line and the infrared magnitudes is seen: as theEW(Ha) decreases the IR magnitudes become fainter. This long-term optical/IRvariability is attributed to structural changes in the Be star's circumstellardisc. The observations include a recent decline in the circumstellar disc andsubsequent recovery. We have witnessed the cessation of a global oscillationdue to the decline of the circumstellar disc. If the present disc growth ratecontinues we predict the onset of another episode of V/R variability by the endof 2006. We have investigated the typical time scales for disc variability ofvarious Be/X-ray binaries and found a correlation with the orbital period. Thiscorrelation is hard to establish due to the difficulty in defining the exactduration of the various activity states, but it is seen both in the duration ofthe disc growth/dissipation phase and the value of the Ha equivalent widthprior to the appearance of asymmetric profiles. These relationships providefurther evidence for the interaction of the neutron star with the circumstellardisc of the Be star's companion and confirms the need of a fully developed discfor the V/R variability to be observed.Comment: 9 pages, 5 figs, accepted for publication in A&

Long-term optical/IR variability of the Be/X-ray binary LS V +44 17/RX J0440.9+4431