equipotential surfaces(ee-kwă-pŏ-ten -shăl, ek-wă-) Imaginary surfaces surrounding a celestial body or system over which the gravitational field is constant. For a single star the surfaces are spherical and may be considered as the contours of the potential well of the star. In a close binary star the equipotential surfaces of the components interact to become hourglass-shaped (see illustration). The surfaces ‘meet’ at the inner Lagrangian point, L1, where the net gravitational force of each star on a small body vanishes; the contour line through this point defines the two Roche lobes. When both components are contained well within their Roche lobes they form a detached binary system. If one star has expanded so as to fill its Roche lobe it can only continue to expand by the escape of matter through the inner Lagrangian point. This stream of gas will then enter an orbit about or collide with the smaller component. The system is then a semidetached binary: dwarf novae, W Serpentis stars, and some Algol variables are examples. When both components fill their Roche lobes, as with W Ursae Majoris stars, they form a contact binary sharing an outer layer of gas. Matter can then eventually spill into space through the outer Lagrangian point, L2 .