The results of experimental studies of magnetoresistance oscillations are described for a HgSe monocrystal with Co impurities of low concentration (<1 at%). It has been discovered that at such impurity concentrations (~1018 cm-3) there exists two types of magnetic oscillations: Shubnikov de Haas oscillations at low temperatures (T<10 K) and magnetophonon oscillations at higher temperatures (T>10 K). The first ones are caused by the interaction of electrons with magnetic fieled inside of a Landau sublevel and the latter --- by the interaction with longitudinal optical phonons. The differences of these oscillations are characterized. A hypothesis of a nature of magnetophonon oscillations in such structures is proposed. Keywords: 3d-impurities of low concentration, zero-gap semiconductors, magnetophonon oscillations, SdH oscillations, optical and acoustic phonons.