The pulsation periods of RR Lyrae stars usually vary with time, and they are often used as probes to study the mechanism behind the variation. After the early discovery that the pulsation period of the RR Lyrae star AX UMa decreased rapidly, in further research, we made multiband photometric observations of this star using the Sino-Thai 70 cm telescope and the 60 cm telescope at Yunnan Observatories, and collected its light-curve data from several photometry sky surveys. The O–C diagram confirmed that AX UMa has a rapid period decrease with a rate of −7.752 0.005 days Myr⁻¹, which indicates that it is the fastest-period decreasing ab-type RR Lyrae star in the Galactic field. Moreover, the O – C residuals contain additional periodic variations. We suppose that the variation with a long period is probably caused by the light-travel time effect as the star orbits in a binary system. The calculation shows that the lower mass limit of the companion is about 1 M _⊙. Combined with the full amplitudes and color indexes, we suggested that the companion is probably a hot subdwarf star. We compared the light curves of AX UMa and those of another binary evolution pulsator, OGLE-BLG-RRLYR-02792, and found that the former shows the characteristics of ab-type RR Lyrae stars, while the latter is more like an extreme long-period c-type RR Lyrae star. However, the absence of a bump in the light curves implies that the mass loss has occurred in the outer atmosphere of AX UMa. The special features of AX UMa make it worth more attention and further observations.

天琴座RR星的脉动周期通常随时间而变化，它们经常被用作研究变化背后机制的探测器。在早期发现天琴座RR星AX UMa的脉动周期迅速减小后，在进一步研究中，我们利用中泰70 cm望远镜和云南天文台60 cm望远镜对该星进行了多波段光度观测，并收集了其来自几次测光天空调查的光曲线数据。所述ø - Ç图证实，AX大熊具有的-7.7520.005天速率秘耳快速周期的减小^-1，这表明它是最快周期减小在银河字段AB型天琴座RR星。此外，O - C残差包含额外的周期性变化。我们推测，长周期的变化可能是由于双星系统中恒星运行时的光程时间效应造成的。计算表明，伴星的质量下限约为1 M _⊙. 结合完整的振幅和颜色指数，我们认为该伴星可能是一颗热亚矮星。我们对比了 AX UMa 和另一颗双星演化脉动器 OGLE-BLG-RRLYR-02792 的光变曲线，发现前者表现出 ab 型天琴座 RR 星的特征，而后者更像是极长的周期 c 型 RR 天琴座星。然而，光变曲线中没有凸起意味着质量损失发生在 AX UMa 的外部大气中。AX UMa 的特殊功能使其值得更多关注和进一步观察。