Despite more than 20 years of study, we still possess only limited constraints on the fundamental properties of SS433. With the exception of the 13d period, orbital parameters are ill-constrained, and hence the nature of both compact object, a neutron star or black hole, or the mass donor are still unknown. However, newly acquired high resolution blue spectra, taken when the precessing disc is closest to edge-on, provide the strongest evidence yet for a massive, A4 supergiant donor, in addition to expanding the phase constraints for further observations of the donor.