diff --git a/ess/lib_tw_elem.py b/ess/lib_tw_elem.py
index 95a08378ef23ecdd407fa5b92aab1570a8675f10..aebc37e6eba903cca6ac56449672c7c26e4109ff 100644
--- a/ess/lib_tw_elem.py
+++ b/ess/lib_tw_elem.py
@@ -1,4 +1,3 @@
-
 '''
     - Classes for TraceWin elements/commands for external manipulations.
     - First double check all the used elements/commands.
@@ -74,6 +73,10 @@
 
         - ADJUST activated.
         - max added to THIN_STEERING.
+        
+      * 2016.08.18
+
+        - Asymmetric aperture of DRIFT implemented.
 '''
 
 #---- Libs
@@ -293,16 +296,22 @@ class DRIFT(ELEM):
         self.apt=float(para[1])
 
         # TW option instances
-        try   : self.apty=float(para[2])
-        except: self.apty=          0.0
+        try   : self.apty     =float(para[2])
+        except: self.apty     =          0.0
+        try   : self.aptx_cent=float(para[3])
+        except: self.aptx_cent=          0.0
+        try   : self.apty_cent=float(para[4])
+        except: self.apty_cent=          0.0
 
     def get_tw(self):
 
         if self.name!='': lin=self.name+': '+self.typ+' '
         if self.name=='': lin=               self.typ+' '
-        lin+=str(self.L*1e3)+' '
-        lin+=str(self.apt  )+' '
-        lin+=str(self.apty )
+        lin+=str(self.L*1e3    )+' '
+        lin+=str(self.apt      )+' '
+        lin+=str(self.apty     )+' '
+        lin+=str(self.aptx_cent)+' '
+        lin+=str(self.apty_cent)         
 
         return lin