How can rotate the gameobject only around the y axis?

The gameobject should rotate only around y axis. PlayerDistance should not be changed. i need this for another script. The gameobjects should look to the player. (when the playerdistance < 10) (my english is bad sorry).

using UnityEngine; using System.Collections;

public class NM2 : MonoBehaviour {

  public Transform player;
  public float playerDistance;
  public float rotationDamping;
  public float x = 10f;
  // Use this for initialization
  void Start()
  {
  }
  // Update is called once per frame
  void FixedUpdate()
  {
      playerDistance = Vector3.Distance(player.position, transform.position);
      if (playerDistance < x)
      {
          lookAtPlayer();
    
          
      }
      
     
  }
  void lookAtPlayer()
  {
    Quaternion rotation = Quaternion.LookRotation(player.position - transform.position);
      transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * rotationDamping);                     
  }