A Rare Cepheid-hosting Open Cluster Triad in Sagittarius

Abstract The following Galactic star clusters may constitute a rare Cepheid hosting triad in Sagittarius: M25, NGC 6716, and Collinder 394. The open clusters have comparable ages ( τ ≃ 80 Myr) and nearly aligned main-sequences and turnoffs after accounting for differential extinction ( A G , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> </mml:msub> <mml:mo>−</mml:mo> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>P</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> ). Gaia DR3 astrometry confirms an association between the three clusters, and importantly, the classical Cepheid U Sgr is a member ( P ≃ 6.ͩ75, τ ≃ 80 Myr), while DR3 data indicate that the classical Cepheid BB Sgr ( P ≃ 6.ͩ64, τ ≃ 80 Myr) could possibly be an escaped member traversing the background field.