Constant speed on elliptical path

So I'm using this script to move my object on elliptical path

 var a : float;
 var b : float;
 var x : float; //offset x
 var y : float; //offset y
 var alpha : float;
 
 
 function FixedUpdate()
 {
     alpha++;
     
     X = x + (a*Mathf.Cos(alpha*Time.fixedDeltaTime));
     Y = y + (b*Mathf.Sin(alpha*Time.fixedDeltaTime));
     transform.position = Vector3(X,Y,0);
 }

How do I modify this script so the object will always move with constant speed on the same path?