Algol, also know as β Persei (26), HR936, HD19356, SAO38592, at R.A.= 3h08m, dec=41° is an eclipsing binary star. Algol's magnitude is typically 2.1, but every two days, 20 hours and 49 minutes it dips to 3.4 for about 10 hours.


In the below Wiki image, the red arrow marks the location of Algol in Perseus.