For the hyperbola $\frac { x ^ { 2 } } { \cos ^ { 2 } \alpha } - \frac { y ^ { 2 } } { \sin ^ { 2 } \alpha } = 1$, which of the following remains constant when $\alpha$ varies?\\
(1) eccentricity\\
(2) directrix\\
(3) abscissae of vertices\\
(4) abscissae of foci