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tokens: Y , X – Z , and Y – Z planes. The inner Sagittarius, the closest Local (Orion), and the outer Perseus spiral arm segments can be recognized in the X – Y projection (a top-down view of the Galactic disc) (cf. Hou Han 2014 , figs 5–7 therein; Nakanishi Sofue 2016 , fig. 7 therein; Xu et al. 2018 , fig. 1 therein; Reid et al. 2019 , fig. 1 therein; Cantat-Gaudin et al. 2020 , fig. 8 therein; Xu et al. 2021 , fig. 1 therein). Stars are indeed located within the extremely thin disc. The Sun is situated within the Orion segment, at the inner edge of this arm (cf. Simonson van Someren Greve 1976 ; Basharina, Pavlovskaya Filippova 1980 ; Cohen et al. 1980 ). Figure 2. Space locations of all 1972 (top panels) and selected 964 (bottom panels) OB stars. The Cartesian ( X , Y , Z ) coordinates are shown, and a dashed line indicates the solar circle and the Galactic mid-plane. The Sun is at (0, 8.2, 0.02) kpc. Fig. 3 gives a plot of los for all and selected stars as a function of the longitude l. A well-defined effect of the Galactic differential rotation is clearly seen. For these two samples of stars – all 1972 and selected 964 ones – the space and velocity distributions have approximately the same properties, as is apparent from Figs 2 and 3 . Figure 3. The heliocentric line-of-sight velocities of OB stars as a function of the Galactic longitude l. Top panel: the velocities of all 1972 stars. Bottom panel: the velocities of selected 964 stars. 3 RESULTS OF CALCULATION To compute the 11 mutually independent unknowns , , , , u 0 , 0 , 0 , , , φ 0 , and λ rad for a given m , we define the weighted sum, S , of squared residuals of measured and modeled (equation 3 ) velocities: where N is the total number of objects, q is the number of parameters to be derived, and is the uncertainty of . The best-fitting values of unknowns are found by fitting that minimizes S . The model depends linearly on the set of nine parameters , , , , u 0 , 0 , 0 , , and and non-linearly on two parameters φ 0 and λ rad . For each model m was fixed and minimization of S by the least-squares fitting was performed in φ 0 and λ rad (that is to say, in the phase of the wave φ 0 and in the pitch angle p at the solar position r 0 for a given number of spiral arms m ). The quantities and are periodic with respect to φ 0 with period 2, while the quantity S is periodic with respect to φ 0 with period . As usual, for a trailing pattern, we chose for the phase φ 0 the values that satisfy the condition (Mishurov et al. 1997 ; Griv Wang 2014 ; Griv et al. 2017 , 2020 ). The surface of S ( φ 0 , λ rad ) and cuts through the S space versus φ 0 and λ rad constructed over selected OB stars for the most interesting m = 1, 2, 3, 4 models are shown in Fig. 4 . A global and several local minima of S with respect to φ 0 and λ rad are seen from our simulation for all models as λ rad increases from 0 to 3 kpc (as | p | increases from 0° to 15°, respectively). Mishurov et al. ( 1979 ) and Pavlovskaya Suchkov