We report the discovery of the classical Cepheid OGLE-GD-CEP-1884 (= GDS_J1535467-555656) with the longest pulsation period known in our Galaxy. The period of 78.14 days is nearly 10 days longer than that of the previous record-holding Cepheid, S Vulpeculae, and thus, OGLE-GD-CEP-1884 can be categorized as the first ultra-long-period Cepheid in the Milky Way. This star is present in the ASAS-SN and Gaia DR3 catalogs of variable stars, but it has been classified as a long-period variable in those catalogs. Based on more than 10 yr of the photometric monitoring of this star carried out by the OGLE project in the I and V bands and a radial velocity curve from the Gaia Focused Product Release, we unequivocally demonstrate that this object is a fundamental-mode classical Cepheid. By employing the mid-infrared period–luminosity relation, we determine the distance to OGLE-GD-CEP-1884 (4.47 ± 0.34 kpc) and place it on the Milky Way map, along with about 2400 other classical Cepheids. We also discuss the potential of finding additional ultra-long-period Cepheids in our Galaxy.

中文翻译：

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银河系最长周期经典造父变星的发现

我们报告发现了经典造父变星 OGLE-GD-CEP-1884 (= GDS_J1535467-555656)，它具有银河系已知最长的脉动周期。 78.14天的周期比之前的纪录保持者S Vulpeculae长了近10天，因此OGLE-GD-CEP-1884可以归类为银河系第一个超长周期造父变星。这颗恒星出现在 ASAS-SN 和 Gaia DR3 变星目录中，但在这些目录中它被归类为长周期变星。基于 OGLE 项目对这颗恒星进行了 10 多年的光度监测我和V根据盖亚聚焦产品发布的能带和径向速度曲线，我们明确证明该天体是基本模式经典造父变星。通过利用中红外周期-光度关系，我们确定了到 OGLE-GD-CEP-1884 (4.47 ± 0.34 kpc) 的距离，并将其与大约 2400 个其他经典造父变星一起放置在银河系地图上。我们还讨论了在银河系中发现更多超长周期造父变星的可能性。