18 Aquilae

18 Aquilae, also known as Y Aquilae, is a triple star system located in the constellation Aquila. It has an apparent visual magnitude of 5.07, which makes it visible to the naked eye. Based on an annual parallax shift of 6.43 milliarcseconds, the system is estimated to be about 510 light-years from Earth.

Whether or not 18 Aquilae is a variable star was debated for decades. In his 1879 *Uranometria Argentina*, Benjamin Apthorp Gould marked it with an asterisk, indicating he believed it to be variable. In 1894, Gustav Müller and Paul Kempf noted Gould's opinion but stated that their many observations did not confirm variability. That same year, Seth Carlo Chandler reported that the star was indeed variable, describing an "unmistakable periodicity" and calculating a period of 4.986 days—nearly four times longer than the currently known period. He assigned it the variable star designation Y Aquilae. In 1898, Edward Charles Pickering and Paul S. Yendell announced that their observations "fail to show any evidence of variation." It was not until 1966 that Robert Arnold Breinhorst established Y Aquilae's variability in his PhD thesis.

The inner pair of stars in the system is a spectroscopic binary with a combined magnitude of 5.44 and an orbital period of 1.302 days. The primary component is a giant star with a stellar classification of B8 III. Because the orbital plane is inclined near the line of sight, the two stars form an eclipsing binary. The eclipse of the primary causes a 0.04 drop in magnitude, while the eclipse of the secondary causes a 0.03 drop. At an angular separation of 0.310 arcseconds lies the tertiary component, which has a magnitude of 6.39. The system also has a high peculiar velocity relative to its neighboring stars.