defining a relative axis of rotation

how can I rotateAround something on the X and Y, but lock the cameras rotation around the Z axis.

so what ends up happening, as I use the mouse to rotate around my camera's Z rotation value slowly creeps away from 0 and its causing my camera to eventually spin about the Z axis. I want the camera to rotate around the target, but remain locked in the z posisition.

so far im certain it has to do with the line where i define the rightAxis, but I dont know of any other way to do this..

     using UnityEngine;
     using System.Collections;
     
     public class camera_control : MonoBehaviour {
         
         
         public Transform target;
         private float v;
         private float h;
         private float w;
         
         private Vector3 rightAxis;    
         
         private float hoSensitivity = 2;
         private float voSensitivity = 2;
         
         
         // Use this for initialization
     void Start () {    
         }
         
         // Update is called once per frame
     void Update () {
         
             h = Input.GetAxis("Mouse X");
             v = Input.GetAxis("Mouse Y");
                 
             //rightAxis = transform.right;
     
     
         if(Input.GetMouseButtonDown(0)) //0 is left, 1 is right, 2 is middle mouse button
             {
                 //raycast for taget change here or to deselect the target.
             }
     
         if(Input.GetMouseButton(1)) //0 is left, 1 is right, 2 is middle mouse button
             {
                 transform.RotateAround(target.position, Vector3.up, h * hoSensitivity); //orbit left/right
                 rightAxis = transform.right;
                 transform.RotateAround(target.position, rightAxis, v * voSensitivity); //orbit up/down
             }
             
     
         if(Input.GetMouseButtonUp(2)) //0 is left, 1 is right, 2 is middle mouse button
             { 
                 //do something when the mouse button was just released
             }
         
             
         }//end update
         
     }//end script